XJ6 & XJ12 Series I, II & III 1968-1992

So confused, V-8 conversion question

 
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Old 09-15-2014, 01:38 PM
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Default So confused, V-8 conversion question

Hi everyone!

So if you recall or saw the post I made in June asking your opinions about buying a 73 conversion, I did it! :-) I bought her for a bit less than a few of you said you would pay and I really enjoy the car! But, now questions arise. :-)

So, we are talking about a slightly modified 350 with a TH350 tranny. I have two issues that I think are independent of each other.

1. I took in to my mechanic (who has now moved to Oregon) because it was hesitating when I floored it. Like it was being starved for gas. So he looks at it, says the two jag fuel pumps were all gunked up and he took them out. Gave them to me in a bag and said he would get new ones but now the the issue should go away and I should be fine for a bit with the other pump. So, first question.. I had 3 fuel pumps?!?! 2 from the jag and one for the conversion?!! Now only one fuel pump?! How many fuel pumps should a conversion have? and, it still hesitates under a heavy foot.

2. After the engine gets hot/I have been driving on streets for say, 30 min, it hesitates and runs poorly. bogged down, idles rough and has occasionally stalled out when idling rough at like 800 rpm while sitting at a light. Also, it did this BEFORE my mechanic took out the two jag fuel pumps. Then when I shut if off, it is really hard to start back up again. When I do get it started it billows one puff of grayish smoke.

After reading in a few 350 forums it seems it could be an air/fuel mixture problem?? But, I wanted the opinion of people here. You guys are always so great.
 
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Old 09-15-2014, 03:20 PM
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I just had this problem with my 350 this Summer and it took me by surprise because it's been a Very long time since I drove a carbureted engine that suffered from (are you ready for this?) VAPOR LOCK, a problem unknown in FI engines because the fuel pumps are in the tanks of those cars.

In warm/hot weather the fuel vaporizes in the lines in the engine compartment on the way to the engine so there's little liquid for the cam driven pump to, well, pump. This is especially an issue if your engine bay is crowded like most Jaguars. (My work truck, oh the other hand, has a Huge engine bay with a relatively small engine (383) and all the gas lines are far away from the engine.)

It can manage to limp along as long as the engine runs, but when you shut it off all that latent heat boils the gas in the lines clear back to the fire wall. When you try to start your car again it has to pump gas for several feet through hot tubing. On a Really hot day this can take some time because these pumps don't do air very well, they're intended for liquid. If the battery isn't in top form you can run it down cranking the engine trying to get it to start.

Probably the 2 pumps in your trunk had been disconnected at the time of conversion as they "weren't needed" since the 350 came with its own. That's why your car started doing this before the mechanic removed those pumps. You had vapor lock at that time and your mechanic should have caught it.

The Very Best way to avoid this Ever happening again is to install an electric fuel pump somewhere well away from engine heat. In fact, that's on my Hit List of things to do to my car this coming Winter.
(';')
 
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Old 09-15-2014, 03:24 PM
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without knowing year of car knowing how may fuel pumps cannot be determined, either has one in spare tire well or two, one being in each fuel tank, Third one i would guess to be the manual pump on teh front of teh engine assuming its a carb'd 350 as it has a th350 behind it!
i would check fuel filter before going much further! if original pumps are gunked up its likely that tanks are too!


Darren

ok just saw that you had a 73, that would have been two pumps! shoulda paid mare attention
 

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Old 09-15-2014, 06:39 PM
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I'd investigate the carburetor. The puff of grey smoke on re-start suggests it's over fueling.
 
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Old 09-15-2014, 10:36 PM
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Default fuel issue

Originally Posted by icsamerica View Post
I'd investigate the carburetor. The puff of grey smoke on re-start suggests it's over fueling.
I would tend to agree with ICS, sounds like fuel pumps have been done to death and nothing has improved.

Just as an example, I had something similar when I bought my car (an 83, XJ6, 350, T400). Car bogged under any hard acceleration and also ran out of puff around 120kph.

Ended up being both a clogged fuel filter and broken secondary needles in the carb (it had other problems so swapped it out completely). Removed a whole bunch of plumbing in the trunk after the tank switching stuff and now just have filter, electric fuel pump, line going forward.

Not saying this is your problem, just an example of how you can have multiple things going on.
 
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Old 09-16-2014, 10:12 AM
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I was so intent on your vapor lock problem I didn't even notice your hesitation when you floor it. I'm thinking you have a failed accelerator pump in that carburetor. If you don't know what that is, its function is to exactly deal with sudden acceleration by squirting a bit of gas into the carb when you floor it, in order to give the engine an extra kick in the pants until the other process catch up. You might look into that.

Still addressing issue 2, here are some questions to ask yourself to know for sure if vapor lock is one of your problems:

Does the car ever do this on a cold start or in the mornings? Even a very warm morning? Vapor lock is seldom is an issue with a cold engine.

Does it only happen in the afternoon, in warm weather and/or in slow traffic, but in similar weather the car runs normally on the open road?

If you manage to keep it alive, does it come out of it after a few blocks of normally moving traffic? Once the engine gets fuel up to the carb it will run like Stink, even if only minutes before it was coughing and hacking and trying to die at a light.

When you shut off the car after a hot run, does it start more or less normally if it has sat for a couple hours before trying to start it again?

My 350 did all these things in July, in the Sacramento Valley, in temps well over 100, in slow, stop and go city traffic.

It's easy to flood a hot engine that's playing hard to start because you get frustrated and embarrassed with the long cranking. Perhaps that's part of the hard starting, and maybe where the gray smoke comes from.

When I was a kid this happened sometimes with one of the trucks during harvest. After idling alongside the combine it bucked and kicked until cool gas worked its way from the tank. But it was only on hot afternoons and only in those slow moving situations. It always started in the morning, and if it was completely shut down on a hot afternoon it started OK after a couple hours.

Our 350s are of about the same era as that truck engine.
(';')
 

Last edited by LnrB; 09-16-2014 at 10:36 AM.
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Old 09-16-2014, 11:20 AM
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It's been a while since I messed with a carb'd engine except for the little critters on my yard stuff.


The two tank, two pump system on the Jaguar is kinda neat, but double the trouble????


And, then add in a SBC with mechanical pump to confuse things???


Well the immersed pump is good, a s it is cooled by the fuel and lessens dreaded vapor lock. As those Jaguars were carb'd, so far so good for the carb'd SBC. Caveat, as long as the pumps and valves are good.


My choice would be to keep it that way and fix any issues.


But, others....


1. Tee the tanks and plumb to the SBC mechanical pump. Simple, but with a caveat, functional.


2. Tee the tanks and add one electric pump and omit the mechanical. That would work. Caveat, electric pumps push great, but pull poorly. Put the electric in the boot.


In the past, I've used an electric pump to boost the mechanical pump. Best located near the tank.


Oh, some just abandon the Jag tanks and place a fuel cell in the boot and an electric nearby to push to the carb'd SBC!!! If the tanks are bad, why not?


Now, Jaguar in its infinite wisdom recognized vapor lock. It's FI engines included an AC powered fuel cooler in the engine bay!! As to non AC FI Jaguar's,
I don't know.


Others with more knowledge than I might.


Way back in old carb'd American cars, vapor lock was possible. All kinds of "cures" from a wood clothes pin on the in in the engine bay to insulation was touted.


As to hesitation... On carb's, remove the air cleaner, look into the throat of the carb. work the throttle. Are there two healthy squirts? If so, the accelerator pumps work, if not, work to be done.


Carl
 
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Old 09-17-2014, 11:14 AM
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Wow, thank you for the really great replies! Lnrb, ICS, yarpos,JagCad and Darrenmb.

I am going to have the carb looked at today and will keep you abreast of any changes that are worthy of a repost. "abreast" I love that word! LOL

But, the carb does give two good squirts when the throttle is engages. Also, when I was underneath I noticed that the fuel line is placed in really close proximity to the exhaust! That scares me, but it is tacked down pretty good.

Anyone else agree with the electric pump in the boot? I really like that idea.

LnrB, to answer your questions, No it does never do this in the mornings. Only after I have driven it for a long period of time. If I go to 7-11 and then home, no issues. But if I go more than 7 miles on the freeway, I can expect the problem. It definitely occurs only when the engine is hot.
 
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Old 09-17-2014, 09:10 PM
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Originally Posted by Socal View Post
[...]
But, the carb does give two good squirts when the throttle is engages. Also, when I was underneath I noticed that the fuel line is placed in really close proximity to the exhaust! That scares me, but it is tacked down pretty good.

Anyone else agree with the electric pump in the boot? I really like that idea.

LnrB, to answer your questions, No it does never do this in the mornings. Only after I have driven it for a long period of time. If I go to 7-11 and then home, no issues. But if I go more than 7 miles on the freeway, I can expect the problem. It definitely occurs only when the engine is hot.
Thank you for those answers.
Two good squirts dismisses the accelerator pump as the hesitation when you floor it. But even if that's not the problem it's good be able to to cross it off the possibilities list with confidence.

The fuel line so close to the exhaust pipe is the answer to your vapor lock problem, I'll just bet! When you're at road speed there is enough air passing by that area to keep the gas liquid. When you're stuck in traffic, well just imagine the heat build-up in the gas line at that point!

The longer you sit idling with so little gas flow the worse the problem becomes until the engine is so lean it simply can't keep on. There is usually still gas in the accelerator pump well so jazzing the throttle sometimes revives it for a while until that's all gone, and then you only hear your engine gasping, hacking and coughing.

That also should explain why it's so hard to start if it sits for only a few minutes when it's fully hot. That exhaust pipe is busy boiling the gas in the line for quite a long distance both ways from that point. Then, as I mentioned, the fuel pump has to bring up liquid from several feet away.

This issue is probably why FI vehicles have the fuel pump submersed in the tank. There's always liquid for it to push (unless you're out of gas of course) and it shoves the gas beyond any vapor bubble very quickly.

I have another carbed car (Chrysler 440) wherein the push rod for the cam driven pump got lost during the last overhaul and one about 1/8 inch too short was put in by mistake. (Not my work!) You can imagine how well that didn't work after about 100 miles. It has an electric fuel pump just ahead of the tank and Well away from either exhaust pipe, and it has Never missed a beat no matter how hot the weather, how long I'm sitting in traffic, or if I shut it off for 3 minutes after driving 50 miles. It starts Immediately and runs strong.

Since these care originally had fuel pumps in the boot it only makes sense to put modern fuel pumps there again. Certainly that will do away with this Stupid vapor lock issue.
(';')
 
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Old 09-17-2014, 09:31 PM
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The stumbling problem sounds like vapour lock which develops inside the engine mounted fuel pump. There's umpteen solutions for it, one of which Chevrolet incorporated in the late 60s-early 70s and consists of a three port fuel pump. In addtion to the usual inlet and outlet, there's a bypass line that allows part of the flow to return to the tank. This provided a continuous flow of fresh fuel through the pump.

Possibly I missed this but what type of carb does the engine have? If it's a four barrel quadrajet, the bogging might be the air valve above the secondary venturis. There's an easy adjustment to prevent the flap from opening too early/too abruptly.

My old Corvette suffered with both these problems, each was easy to fix.
 
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Old 09-18-2014, 10:19 AM
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Default Get it fixed at SMI (Sean Murphy) in Huntington Beach

Sean fixed my '74 383/700R and '71 383/200R. He is the carb King on the west coast...
 
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Old 09-18-2014, 11:23 AM
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Mikey, it is a Edlebrock 600cfm. What is this easy adjustment you speak of? :-)
 
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Old 09-18-2014, 11:35 AM
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Originally Posted by Socal View Post
Mikey, it is a Edlebrock 600cfm. What is this easy adjustment you speak of? :-)
Doesn't apply on an Edelbrock carb, just Quadrajet.
 
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Old 09-18-2014, 02:44 PM
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Thanks again for all your help, gentleman. I think we have established vapor lock as the culprit! So, now that I am looking at electric fuel pumps, I have no idea what to buy! For example, there is a Edlebrock 80GPH (what is GPH?) with 45 psi for $555. OR there is a Holly one that is 110 GPH and 7psi for only $212. How do I know which one to buy? If it matters for the electric fuel pump, I have a chevy 350 bored .60 over with a TH350 tranny.
 
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Old 09-18-2014, 02:54 PM
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GPH= US gallons per hour flow at a given pressure.

Edelbrock carbs don't like much more than about 5psi inlet pressure, beware.
 
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Old 09-18-2014, 04:49 PM
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I have a Pierburg fuel pump in the boot/trunk , it runs 4-6 psi from memory. They are original equipment on many cars and seem to have a good rep.

Catalog here if you are interested Motorservice: Fuel supply

Brand really doesnt matter, just what you are comfortable with. As Mikey says carby=low pressure pump.
 

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Old 09-18-2014, 06:18 PM
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Just stick with the normal Chevy mechanical fuel pump on the engine. They've been installed in million of cars and have been reliably in use for over 50 years. If the mechanical pump you have is bad for some reason, replace it. You can manage vapor lock by routing fuel the line properly and using heat shielding where necessary.


http://www.summitracing.com/search?S...hields%203%2F4
 
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Old 09-18-2014, 06:49 PM
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Originally Posted by icsamerica View Post
Just stick with the normal Chevy mechanical fuel pump on the engine. They've been installed in million of cars and have been reliably in use for over 50 years. If the mechanical pump you have is bad for some reason, replace it. You can manage vapor lock by routing fuel the line properly and using heat shielding where necessary.


http://www.summitracing.com/search?S...hields%203%2F4
Not so much on today's fuel with seasonal RVP adjustments. Cars with fuel injection are pretty much unaffected but at the expense of carburetted cars like the OPs (and mine).
 
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Old 09-18-2014, 07:16 PM
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on various vehicles I have used a Carter brand pump. It was good with the Webers, plenty of volume and low pressure. Google it!
 
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Old 09-18-2014, 08:00 PM
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Originally Posted by icsamerica View Post
[...]
You can manage vapor lock by routing fuel the line properly and using heat shielding where necessary.


http://www.summitracing.com/search?S...hields%203%2F4
Thank you, isc,
I'll look into that shielding for sure. This would be Loads easier than trying to sort out what they did during the conversion and trying to undo some of it.
(';')
 

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