Ok, went to a friend of a friend's house to look at his 88 4- cyl-

The truck runs rough until it warms up, and makes a high- frequency 'spitting' noise back through the carb. It eventually settles out- until you goose the motor, and then it does it all again for maybe 5 minutes... and then settles down again.

He thought it was exhaust valves sticking, but it's not making much valve lash noise. He's replaced the VLA's with no change in symptoms. We checked the EGR- it seems to be closing, but when it's open, we get symptoms very close to the idle problem- but with a different noise!

My guess, after much yakking and not nearly enough beer, was that it's a couple of the little jet valves (or whatever those dumb <img src="/forums/images/graemlins/butwiggle.gif" alt="" /> things are. It's just too quiet for a main valve problem.

whaddya all think?

Toby

I'm thinking Reed Vavle or Air control valve may be defective.
It's the tube that comes off the exhaust and feed's air back into the air cleaner. It might be sticking open until warm or the vac line might have a issue.

Check the choke while your at it.

<img src="/forums/images/graemlins/patriot.gif" alt="" />

is it a backfire through the carb? timing maybe?

Oops- we did check the reed 'gulp' valve back to the air cleaner- that wasn't it.

No, it's a really steady, very loud hissing in the carb primary barrel. Sounds like a REALLY big cat with a little bit of a cold (it's a 'wet' sound) and it doesn't pulse much.

Any other takers?

Thanks,
t

http://www.fcproducts.com/parts_catalog/CRY290.htm

I think this is the problem <img src="/forums/images/graemlins/lol.gif" alt="" />

Check the engine temp sensor and refresh your memory by looking at page 14-20 troubleshooting section. Carb PDF file

Are you sure it's not a disconnected or cracked vac line. Pull them all off one by one and check the underside.

Did he take the spark plug wires off. Check firing order FIRST. Vac line check second (check dizzy advance can for holding vacuum). Check egr vac can for tight. Check float bowl setting - may be low. The sound from the primary is fairly normal sounding from your description. Would you say it's equivalent to a 6psi air leak in frequency. Add the wet gas sucking sound from the idle jet for the liquidity. The spitting could be mis timing firing from cross wired ignition wires or cracks in the dizzy cap, or carbon track arcing paths on the inner surface of the cap, or even induced spark caused by running ign secondary wires to close and too parallel for too long a distance.

To check the jet valves for lost heads, remove the valve cover and set the timing on tdc for #1. On the intakes with closed valves, back off the jet valve lash aduster and see if the stem pops up too far. Turn the crank one compete rev to tdc on #4, and the rest of the intake valves should be closed. Repeat the jet valve inspection.

Check the brake vacuum line for cracking at the manifold nipple on #4. It's in close proximity to the heat from the water passage in the mani from the head, and it can get internal cracking that leads past the clamp to atmo ambient, causing a big vac leak in #4, and lean misfires. Lean misfires are worse on a cold engine, and worse under engine acceleration, especially under load.

The vac lines around the egr on the carb heat rot early. Check close.

Happy wrenching....

Toby: Does this hissing feel like pressure coming from the carburetor air horn?
Conner

Conner-
yeah, it really does feel like it's spitting back.

What's so strange about the whole thing is that the motor will settle down after about 5 minutes of idle, and then the symptoms will reoccur if you rev it above 3K rpm. Then, in about 5 more minutes, it settles down again. And idles very quietly and smoothly!

good idea on the jet valves- I'll have him try that.

Thanks, all- I'll keep you posted.

Toby

And while you are at it, ck. for a plugged exhaust.
Conner