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Wedge Help in solving TR7 lift-off backfire

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sammyb

Luke Skywalker
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Okay, I'm baffled! I need help.

Here's what happens: When lifting off the accelerator, the TR7 backfires, seemingly through the carbs. The higher up the RPM range I let-off, the louder the backfire. (Going full throttle up to 5000RPMs lets off a good old pop!)

Here's what I know or have found-out.

-The spark plugs look perfect: just a little tan on half the plug...no white deposits or charring.
So it doesn't appear to be a lean condition.

-The temp gauge reads dead in the middle.

-Both Stromberg carbs are new as of '99, which was only 1500 miles ago. They still look new, with clean pistons which move up and down perfectly. I have ATF in the dashpots.

- The hot air return hose (exhaust manifold to carb air box) has totally disintegrated, but all other hoses and emissions parts are seemingly intact and not leaking.

- I tried to reset the bypass valves. removed advance from distributor and plugged it, and idle only went to about 1300RPMS. In following the procedure in the manual for resetting, turning either adjusting screw in couldn't bring the idle past 2000RPMS.

- After adjusting the bypass valves, it seems there's an intermittent pop/backfire at idle. So I must have done something stupid!

- The previous owner filled the gas tank the day I bought the car (10 days ago.) So the gas is new. I have not changed the fuel filter.

Any ideas? Where should I look.

Feel free to ask questions or give suggestions.

Thanks!!!!!
Sam
 
Although I don't own one of these flying doorstops, I have a friend who had one that did the same thing.
It turned out to be a problem with the "gulp valve" (which is also called the bypass valve).
As far as I understand, these valves open a bit during decel, allowing extra air into the intake manifold to reduce emissions. I recall that MGBs have a similar device.
Also, be sure you don't have any small exhaust leaks. This can also cause backfire on over run with some cars of this vintage.
G'luck.
 
Nope...haven't had much of chance. I think I'm going to remove the carbs and block off the bypass valves with gaskets (as suggested.) If that doesn't work, then I'll listen for exhaust leaks somewhere.
 
Be very gentle with the diaphrams when you do this (in case you want to reuse them). I pulled mine last summer and found that both leaked. I did not have the same problem you are experiencing though. I had very little engine breaking when I let off the gas. Different car after I got it back together.
 
I took one of the bypass valves off (the front carb) and it looks absolutely perfect. I didn't bother taking the back one off (a hassle) because the carbs basically look new inside and out.

I didn't quite adjust the valves properly, so now I have to go back and do the adjustment process, but the car is still popping.
 
Sam,
I started getting a similar type of backfire after taking it around the block last few weeks. (No Ins. so that is as far as I dare). I rebuilt the engine in the fall, 78 TR7 2L, and as I have torqued down the intake manifold to the specs in the book, the backfire is gone. I didn't do anything else to it. Perhaps you could check that.
smile.gif


By the way..... Who decided to put those bolts where they are????
mad.gif
 
<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>By the way..... Who decided to put those bolts where they are???? <hr></blockquote>

I decided a while back that British engineers are sadists with a warped sense of humor. Another prime example is what they did with the cold start injector on FI 7s. It's under the air plenuum, and the bolts are so close to the bracket that you can't get a socket over them. Have to use a 10mm open end wrench that's no more than 3-4" long. It's a Bosch system, and the Germans used allen bolts instead....
 
Originally posted by sammyb:
[QB]Okay, I'm baffled! I need help.

Here's what happens: When lifting off the accelerator, the TR7 backfires, seemingly through the carbs. The higher up the RPM range I let-off, the louder the backfire. (Going full throttle up to 5000RPMs lets off a good old pop!)

Sam,
My '80 TR-7 Spider, with an original engine wiht 65,000 on it, does the same thing. I run my daily here in Maine, in all weather, so it's not a build-up problem or bad gas problem. I'm told that under serious load there's a fine sparking out the exhaust, too.

All British cars of this vintage used a gulp valve and when they go bad, you do get the backfire. But replacing the valve alone does not end the problem forever. It seems that many British cars of this heavy emission control era do this, regardless of the fix.

If there's any leakage in the exhaust, you can get backfiring, too. Check at the manifold and at the studs.

Good luck,

Jeff
Click to expand...
 
Hey Sam,

Just a note that may be of interest. My TR7 ran fine without any backfiring issues, however, I just removed the old and hacked up system from my car and founf the gasket between the manifold and the flange had been leaking. Never heard it leak, however looking at the gasket it was pretty obvious. May be some where else you might want to look.

Steve
 
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