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TR6 Interesting TR6 running issue

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sd80mac7204

sd80mac7204

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I'm working with a guy who's problem has me totally stumped and I was hoping that some body here might have experienced this before.
74 TR6
130# all cylinders on a compression test
timing is 4 deg after TDC
Fuel filter was changed
needle and seat valves were changed
engine has spark
The engine runs roughly and stutters and backfires. He removed the air cleaners and says that when he places his hand over the rear carb intake that he feels suction and the engine stalls, when he does it on the front carb, he feels no suction and the engine runs smoother but his hand gets covered in raw fuel. He's checked for vacuum leaks and intake blockages and found nothing.
What am I missing? Any thoughts?
 
First question: how did the engine run *before* the work? (fuel filter and carbs)

I'm thinking you've got stuck valve(s) blocking suction.

Tom M.
 
The car had run well before being put into storage a couple of years ago.
 
Thanks. So it hadn't run recently until *after* the carb/filter work was done?
 
oops. My mistake. Car ran well and he parked it. It ran poorly when he got it out with these symptoms. He did the work trying to fix the problem with no success.
 
Thanks. I'm sure no expert, but I'd try removing the valve cover, check and adjust all the valves, and then maybe using a solvent (Seafoam?) to free anything that's stuck.

I'm guesstimating that crud/carbon in a cylinder, left unmoved for a few years, has frozen a valve. Just an idea anyway.

Tom M.
 
Thank you! I'll look into that!
 
:encouragement:
 
I just had a similar issue and it was some crud in the needle/jet assembly of the front carb. First I took the float bowl off and everything looked fine, wasn't until I unscrewed the assembly and took it apart that I discovered and fixed it.

Not saying that is it, but worth checking the carbs and maybe at least shooting some carb cleaners through them if car has been sitting. Also check the carb diaphragm for tears.
 
I would think that the compression test would rule out the valves sticking especially if they are all very close to 130 lbs. If a valve was sticking your test would be way off one one or more cylinders. However I would still do the Sea Foam fix. Use one of those thin red straws and shoot the foam between the valve springs to get it into the the guides. Maybe do it a couple of times and let it sit overnight. Sounds like your only running one one carb. I'd do what glemon said while soaking the valves and in fact I'd do both carbs. If one has crud in it the the other probably too. Then re-balance the two carbs which maybe the actual problem anyway.
 
sd80mac7204 said:
timing is 4 deg after TDC...Any thoughts?
Click to expand...
Double check how you're setting this...disconnect & plug both advance and retard vacuum lines...then set 10-12 deg before TDC. WIth my GT6's 6 cylinder, I've found it idles best leaving the retard line blocked.
 
Just a follow up to the original post - does anyone have a thought why he "feels no suction [at the front carb] and the engine runs smoother but his hand gets covered in raw fuel."

That's why I was thinking valve problem.
 
Given that this car sat for 2 years ..... fuel evaporates and carbs become the problem.

What was done to the carbs? Jet and seats or a full rebuild, to include diaphragms, floats and seals?

With air cleaner off and looking at the inlet of both carbs while idling, do both pistons rise the same as rpm's are increased?
 
+1 on what Marv said. Old gas in itself will cause the engine to run poorly. It can also form a deposit that can cause the needle valves to stick and corrosion can form on the metering needle. The carb diaphragms can also become stiff and effect the air valve movement. Two years storage can cause problems in the fuel system (from tank to carbs), especially in a damp climate if a stabilizer wasn't used.
Berry
 
This doesn't sound like a difficult problem to solve.

First, it seems abundantly clear that only one carb is working, for whatever reason. The two sides of the manifold are connected together by a cross pipe, so if you cover the one nonworking carb, the other supplies all cylinders. Since the working one is OK, it's no surprise that things smooth out.

Second, good compressions rule out a stuck valve, even though that seems likely based on the other symptoms.

Third, if one carb is spewing gas, that's an obvious problem. Even though the needle and seat are new, they still can get stuck open. The float level can also be wrong. Old, bad gas won't help the car run well, of course, but won't make the carb leak fuel into the intake.

So, I'd pull the questionable carb, take it apart, and see what's happening. A bad diaphragm is a real possibility, but I suspect there is more.
 
Lack of apparent vacuum at one carb makes you wonder if that throttle plate is completely closed, but I don't get the gasoline expulsion if that is the issue.
Bob
 
Sarastro said:
This doesn't sound like a difficult problem to solve.

First, it seems abundantly clear that only one carb is working, for whatever reason. The two sides of the manifold are connected together by a cross pipe, so if you cover the one nonworking carb, the other supplies all cylinders. Since the working one is OK, it's no surprise that things smooth out.

.
Click to expand...

That cross pipe theory may be a little optimistic as far as transferring an adequate amount of fuel from one carb to the cylinders normally fed by the other carb.
The internal diameter of that vacuum pipe is only 1/2" in diameter...It's there to provide access to manifold vacuum for things like a PVC valve, an EGR valve, an anti run-on valve, a diverter valve depending upon year... and for all models the brake servo.
 
Forgot about the diaphragms. Check them to make sure they are not cracked or hard, replace if needed then re-balance the carbs. One carb can easily pull more vacuum then the other if one is turned up higher then the other one. Using a Unison you can watch the vacuum increase as you turn up the respective carb.

As far as the cross pipe it's main use is to make a connection between the two carbs to keep a constant vacuum in both carburetors. Otherwise after number 1 cylinder intake valve closes there is no vacuum and the front carb shuts down completely. Then number 5 wants fuel and the back carb has to wake up to give it fuel. Then shuts down when vacuum disappears and now # 3 cylinder wants fuel and the front carb wakes up and so on. With that balance tube in place there are no carburetor lag or sleeping and no loss of time so no loss of power. Hope I explained it well.
 
I'm going to have to think about that theory...there's things going on in other cylinders creating vacuum for both carbs.

Anyway, as far as the dysfunctional carb, often a loosely clamped throttle shaft coupling is the culprit.
 
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