AQ 151 Problems

[thostorey]

Hi Folks, new to the forum and I NEED help. Here are the details:
AQ 151 1988
Twin Solex Carbs. Ran ok last year (October) but the odd miss that cleared after running at an open throttle.
Good strong spark - all ignition components new and tested.
Compression is 175 across the 4 cyl.
licensed mechanic set timing bang on
Runs at idle but backfires, smoke pops out rear carb, engine dies
Noted in the past that the rear two plugs have carbon deposits like they are running too rich. However, last year that did not cause an idling problem
Mechanic will remove timing (chain?) cover and see if it has slipped.

Any and all info/suggestions welcomed! Tom

 

[Sixpence]

Hi Tom and welcome , just a suggestion until the real mechanics sign in , but have you checked the exhaust ? , I had a similar problem with a diseasal a while back and spent ages trying to trace all sorts of possibles , only to find the exhaust was blocked , just a thought

 

[stephenzoie]

Had a similar problem years ago on my Volvo 130. it was caused by the camshaft lobe wearing on an exhaust valve. The engine ran fine at low revs but blew back through the carb on higher revs. It was easy to spot on an overhead valve engine as the exhaust valve was not opening fully.
It may be harder to measure on an overhead cam engine but it's worth taking the valve cover off to have a look.

 

[Its_Only_Money]

Sort the mixture out on the rear carb, if it has been running very rich for a long period then you may have a carbon build-up in 3 and/or 4 and you may need a de-coke, the carbon build-up may be causing pre-ignition and hence the backfire into the carb. Important you sort this as fairly good fire risk!

Timing "slip" possible of either the valve train via the belt slipping a tooth (but would give other symptoms), never heard of a chain slip but they could break but that would be obvious. If he has set the ignition timing "bang on" then he would have compensated for any slip elsewhere in the distributor drive....

The other thing to check is that you haven't ended up with too much ignition advance (most easily caused by setting the advance using the wrong procedure for the engine), this would also cause your symptoms and it could be that cyl 3 or 4 just backfires first....

Either way the rich running on 3 and 4 needs sorting so you may as well get that sorted and work from there.

 

[Col]

Having checked what others have said. A couple of other suggestions.
1/ check carbs are balanced
2/ make sure 3&4 plug leads haven't been swapped over.

I don't think you have valve/ cam probs as you have good compression (but don't rule it out)
Let us know what you find.

 

[thostorey]

What are the odds of this?? It now runs properly after the THIRD new condenser! I was an electronics tech for a number of years and I have never heard of such a failure rate!

Anyway, I may need a bank loan to pay for the hours the mech put in but at least I can get on the water, Thanks to all who answered my plea for help. Tom