68 roadster w/rough idle and backfire out the carbs

I am a 22 year old jobless graduate waking up a (what seems to be controversial on here) Primrose 68 from a twelve year sleep. So far it has been a great project keeping me from facing the 'real world' or any big decisions I'm supposed to be making. But I can't shine the yellow paint anymore until it runs without burping and farting.
Here's the situation:
The car idles fine for a few minutes or so, but then the idle slowly falls and it becomes rough and uneven. Every time I accelerate after it has fallen to this state, the car backfires and hesitates. After running on open road for a few minutes it clears its throat and drives fine. This has made it almost impossible to set the idle and at intersections I have to keep my foot on the gas after the first couple minutes in order to keep it running. I have tuned and retuned the carbs, timed it both stroboscopically and statically, run Seafoam through the carbs and gas tank, drained the tank twice, run at least a 6-pack of lawn-mower filters until the filters were clean... My question then is, are these symptoms of bad timing, or carburetion, or dirty gas? Thanks, and I'll post a picture of it for the Primrose fans when I get the interior done. Brian Gould

I'd suspect a choke and/or mixture problem, first.... what do the plugs look like? You may need to pull 'em right when it's doing.

Also, you could (should) put one of those clear filters on it to see if it's getting clean gas.

Is it missing? If so, pull the plug wires to isolate which cylinder is missing.

Which carb(s) are you running?

And where did you set the timing? After posting all the above, I got thinking about the backfiring, and how that usually seems to be timing/ignition related.

Welcome aboard Brian!! It does sound a little confusing!! Obviously, you have replaced your points and condensor, right??

Oh yeah, have you checked for vacuum leaks?

At least I'm not the only guy in his 20's delaying the enevitable by working on an old car. Unfortunatley I do have a job and I am going to school. However it is tech school, which makes it fun.

One question, how are you able to fund your project with no job?

I LIKE primrose

Brian,
You may be fighting with worn-out SUs. If the throttle shafts are leaking, it's going to be very difficult if not impossible to tune properly. Get some carb cleaner and spray it around the ends of the throttle shafts. If the engine idle changes, then you've found your vacuum leak. The cure is to rebush and install new shafts, or to ream the carb bodies out and install new oversized shafts (+0.005" as I recall...). If you're uncomfortable with this kind of work, then your options are to either buy new carbs ($$$$) or to have yours rebuilt ($$)....

You can use the carb cleaner trick to find vacuum leaks in other places as well -- around the intake manifold, around the carb bases, etc....

I prefer WD-40. It's slightly less flammable.

WD-40 should work as well -- I think it's actually the propellant that gets sucked in and burned rather than the product itself. I usually use carb cleaner because stuff usually needs cleaning anyway... You bring up a good point though -- for any and all car work, have a fire extinguisher handy!

Is it possible that it is vapor-lock? When at idle, the heat coming off the manifold may be boiling the fuel in the lines or float bowl.

Does your car still have the heat shield between the carbs and the manifold?

Gee, Brian, I am the only one here that does not like Primrose, so don't blame the whole BBS! You backfiring sounds to me to be caused by an overly lean mixture. Try pulling the choke out a bit to see if the problem gets better. If this helps, I'd look at hte jet level, the float level and/or throttle shaft bores

Thanks everyone for the responses, and for the welcoming Gary.
I am running SU hs4's that have recently been rebushed by the previous owner. they feel tight with no freeplay. I have replaced the points and condenser. It is running 10 degrees from TDC. Also I was not able to locate any vacum leaks, at least not on the areas accessible with a can of WD40. I have been using those cheap clear NAPA lawnmower filters, and the gas now runs cystal clear through them. Nonetheless, I know much can get through them and I am contemplating a removal of the tank with the full vinegar and Bill Hirsch sealant treatment. Has anyone else tried this?
Vapor lock is intriguing since the idle only goes bad when its been sitting at an intersection for long enough to let the heat build up under the hood, though the car does still have its heat sheild and raising the hood has no effect.
Other ideas I have had include a sticky valve, or a small leak in the exhaust system that I am not finding. But would such a leak cause backfire through the carbs?

Brian, I know that I already said it, but a lot of times a backfire is in that the points are lose, old and poor or something like that!!! The other serious possibility is the condensor!! If you have time for a story, here we go!!! :o)
As a lot of you know, I just enjoyed a few days of my vacation in an British Car shop in Boise Idaho!! Loma, the owner, let his helper fix this MG that came in with a 'run's terrible and backfires when it gets warm' problem!! Carl, a Jag expert and MG novice, talked with the owner and determined that it was running too lean, or something to do with carburation. He rebuilt the carb and sent it on it's way. Now of course the customer lied to him (as most customers do) that the points and condensor , cap and rotor were new!! What he neglected to add was '15,000 miles ago'!! So anyway, Carl 'fixed' it a couple of times, and it came back again!! Loma, a 68 year old fabulous guy, looked at it this time!! He did like most of us do, pulled the distributer and placed it in the vice!! Pulled the cap, points were shot, in fact the ground wire was broken and flopping around like a chicken with a broken leg, replaced what needed to be replaced and had a happy customer!! Afterwards we were talking, and he agrees with the concensus on here, only touch the carbs after you have done a major tune up!!!
Sorry to be so long winded, but I found Loma a wealth of information!! At 68, he used to have the MGB dealership in Boise, and now just does it because he loves it!! By the way, he still drives a race car!!

No, an exhaust leak backfires thru the exhaust!! The other thing, is have you set your valves?? A tight intake does backfire thru the carb!! When it is warm is when the clearances are important

It still sounds like a timing problem to me!

Brian, 10 TDC at what RPM? I've had my best luck setting it by the book, which is something like 20 or 25, at 2200 RPM. I can't remember the exact spec.

And what do those plugs look like?

I would do the following:

1)Check the points. If your running a sports coil they can burn up points quickly.
2)Reset the valve clearances to 0.015 cold or 0.013 hot.
3)Your carbs seem to be running lean, enrich the mixture!
4)Do you have your heatshield installed?

Damn right, Primrose looks good!! Gerry can't help himself.


JTB
'69 Pale Primrose Yellow Roadster

Maybe he is colour blind!! :o)

brian,,,first get a job and then get a weber carb set up..throw away the su's and attach the webber..if your engine is in good shape the weber will take you to the next level of uphoria.

I'd start by double checking your points gap - it's pretty easy to get wrong (I speak from experience!). Next, try advancing the timing a few degrees and see if that helps. Your symptoms sound a lot like mine when the timing was just a bit retarded.
I've got a Weber carb....if your problem is in the ignition, changing carbs won't help. And ignition is way cheaper to fix!!

Don't listen to Jerry tonite!! He is weird!!

And forget about that "job" thing....it's not all it's cracked up to be...

on second thought...it can be a timing problem....although a weber will takecare of a lot of carb headaches.