Runs Rough Under Load

[N233TX]

Midget 1500/Weber Carb Runs great at all RPM in shop. Intermittent rough running under load, especially uphill. Stumbles big-time under quick acceleration. Timing, points, rotor, valves, plugs, FP A-OK. Electric choke carb adjusted per manual including accelerator pump. This engine will sometimes completely fail to start when it gets hot enough. The coil has less than 500 miles and ten years on it. No vacuum leaks apparent.

Most of this seems like a fuel/carb issue of some kind but it has been suggested that bad plug wires and other ancillary parts could be the culprit.

Since this is a problem under load, what does this suggest? I know aircraft plugs can misfire under high pressures but can voltage be momentarily lost elsewhere in the ignition circuit when you punch it going up a hill?

I have absolutely no problem getting new wires and maybe an electronic distributor but before I start throwing parts at this, can somebody tell me if there is further carb trouble-shooting to be done.

This problem has gotten progressively worse since carb installation a few years ago. Tailpipe looks rich but plugs are perfect.

TIA, Jim

 

[bug_sixty]

coil??? does that and only gets worse..

 

[Guest]

make sure the intake is tight, the bottom rear (the one that's hard to get to) sometimes comes loose.

Yes, bad plug wires can/will do that. Could be fuel delivery/vapor lock too (hot start). Make sure your tank is being vented properly as well. No, leaving the return line open you are no longer using with the Weber isn't good enough. Make sure you have the right gas cap and it's working properly. You already have an electronic distributor if it still has the stock one (albeit not the best one). It wouldn't make sense to buy a new distributor and still use old unknown condition wires.

If it were me, I'd put new wires (Accel V8 solid core universal kit, you can make two sets and they should be around $20) and an electric fuel pump. You may be having some vapor lock issues. When I had the DGV on mine I had trouble after letting it sit after running. It would crank and run but kill a short time later. I was able to "cure" the problem by just letting the fuel pump run and push the vapor out. Put the pump in the back by the tank.

What is your timing set at?

 

[N233TX]

Thanks for suggestions. Will try all...one at a time. Starting with the coil. Will post solution when and if one is determined. Timing set to US spec.

I have heard a different jet in the Weber cures similar symptoms. Any specificity on this? Elevation hereabouts is around 1,000' MSL.

Jim

 

[nomad]

Never heard of anyone having good luck with the Weber downdraft on a 1275. Probably the same on a 1500. As mentioned you may need to change some things on your carb. I had one on a 1275 engine in a car I acquired that the PO had been modifying. Car ran great but barely beat 20MPG.

Kurt

 

[N233TX]

It was the coil. Popped on a spare (removed from car in last century) and the problem went away. Astounding. My only experiences with coil failure were normal ops followed by completely nothing when hot, then miraculous regeneration after cooling off. Never experienced loss of spark under acceleration/load. It just seemed so fuel-related!

Hope the Weber lasts longer than that 1,000 mile coil...

Again, thanks to all who responded. Jim

 

[drooartz]

:banana: Woohoo!

These cars present many ways to learn new things. Some good, some frustrating.

 

[Guest]

It was the coil. Popped on a spare (removed from car in last century) and the problem went away. Astounding. My only experiences with coil failure were normal ops followed by completely nothing when hot, then miraculous regeneration after cooling off. Never experienced loss of spark under acceleration/load. It just seemed so fuel-related!

Hope the Weber lasts longer than that 1,000 mile coil...

Again, thanks to all who responded. Jim

What you are describing is the symptom of another problem, not the coil itself. Excessive resistance (i.e bad wire) will cause a coil to find another path to ground. Once it does, it will usually take that path even after the initial problem is fixed. There is no doubt your old one was bad, but it just didn't go bad overnight. You have a problem somewhere else that caused it to go bad. (defective crap being sold as new parts not withstanding). There is a chance this will happen again. Seeing you say everything else is good (I'm assuming new) but the wires, you owe it to the car to change them. Piece mealing ignition, electrica, brake or fuel systems on a 30+ year old car is asking for headaches. Do it all at the same time and you're done with it for several years. Screwing around with it here and there only assures you will be doing so again in the future and a lot more than you'd like to.

 

[Trevor Jessie]

Watch the wires while the car is idling outside in a dark location. You will probably note some arcing.

 

[N233TX]

New wires on the way. Appreciate the explanation. Will temporarily install bad coil after wire installation to see if this was, indeed, the cause.

Still learning about this car after 32 years ownership... Jim

 

[Pythias]

Never heard of anyone having good luck with the Weber downdraft on a 1275. Probably the same on a 1500. As mentioned you may need to change some things on your carb. I had one on a 1275 engine in a car I acquired that the PO had been modifying. Car ran great but barely beat 20MPG.

Kurt

I am having fantastic "luck" with a downdraft Weber on my 1275. The thing is, it's not LUCK. the biggest difference i think is that I took mine to a dyno shop where they know their business. They had all the knowledge and the jets to get it right. I have no off idle stumble, the idle is perfect after the car warms up and unlike SUs is the same no matter the ambient or running temperature its the same. I'm getting a touch over 30 mpg. While this is not as high as SUs, it's still perfectly acceptable (to me) especially since I don't have to fiddle the carbs anymore.

 

[nomad]

Well, yes, Bill but that is probably what is needed to make the Weber downdraft run right on a spridget. I have one on a Datsun pickup that runs far better than the original carb and gets better gas mileage. Carb was right out of the box. Some guys with MGB's seem to get by OK but for Spridgets I would think they need a little massage. Nothing wrong with the Weber just the tune.

Kurt.

 

[Guest]

. Will temporarily install bad coil after wire installation to see if this was, indeed, the cause.

Might be a good idea to re read my last post before you do that.

 

[Guest]

Never heard of anyone having good luck with the Weber downdraft on a 1275. Probably the same on a 1500. As mentioned you may need to change some things on your carb. I had one on a 1275 engine in a car I acquired that the PO had been modifying. Car ran great but barely beat 20MPG.

Kurt

I've had almost every induction setup for the 1500 known to man and I couldn't get higher than 25 mpg if I pushed it off a cliff with the ignition off.

 

[JPSmit]

I've had almost every induction setup for the 1500 known to man and I couldn't get higher than 25 mpg if I pushed it off a cliff with the ignition off.

Yeah, but that's not the carb, it's your right foot.

 

[Guest]

Yeah, but that's not the carb, it's your right foot.

I don't drive it that hard.