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1987 Supersprint carburetor adjustment


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I have a quick question before I have a shop work on calibrating and cleaning my Weber DCOE 40s.

 

Is it normal to have the Crossflow engine hesitate and spit when throttling up from idle? Mine is fine when idling and when above 3000 RPM, but in between I get a lot of popping, spitting and hesitating. Some of it is undoubtedly caused by the aggressive cam, but my hope is that the carbs can be set up so it is smoother on the road and in traffic.

 

Thanks/Anker

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I've had three cars with DCOEs, including a Crossflow similar to yours (1700cc, Kent 234 cam). None of them had the issue you describe. The closest was a built Alfa that used new casting DCOEs with progression holes that didn't work well with the engine, and would go through a rich/lean/rich cycle when cruising around 3000rpm.

 

Have you already verified the basic settings are correct? Float height, various jets and emulsion tubes? Have you synchronized the carbs? Checked for air leaks when they attach to the manifold? Confirmed timing is correct? Occam's Razor doesn't seem to apply to DCOEs, so even if you find several of these things out of whack, you could still need a rebuild.

 

-John

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I have checked and adjusted float heights, adjusted the idle screws as specified, and synchronized the carbs. The first thing I did was to replace the gaskets between the carbs and the intake manifold with the Swedish ones (can't remember their name). There definitely aren't any air leaks. I also replaced the distributor and coil (Lucas electronic ignition) with a new distributor and coil from Burton Power with an Aldon Ignitor. The ignition timing is spot on. In short, I have done all that I feel competent to do.

 

It is better than when I purchased the car, but not to my satisfaction.

 

The information I needed is that this is not normal. It will be going to a specialist shop that knows how to set up Webers next Thursday.

 

I appreciate the reply!

 

Thanks/Anker

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Someone here may know the correct jetting for the Supersprint spec engine. If not, I'd try BlatChat for that info. Someone in your car's past may have tried to "improve" things by playing around with chokes, various jets, or emulsion tubes, and screwed things up. I've even heard from someone who discovered a random PO had changed the emulsion tube in just one carb, creating a mismatch. Always good to know if your starting point is correct.

 

-John

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As another point of reference, I can say my 1700cc SuperSprint X-flow Caterham engine never did that. Lots of cool popping on deceleration. Maybe BBall will offer his thoughts, he has chased many X-flow engine demons.

 

And, on a related mater, both BBall and I had huge problems with our carbs due to worn engine mounts. The engine moved (a lot) as a result of the worn mounts, the air filters poking out of the bonnet were then forced against the edge of the bonnet, causing a vacuum leak and resulting in poor running. This only occurred on left hand turns. The X-flow mounts are a high wear item and should be swapped out every 5K miles or so, FWIW.

Edited by Kitcat
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Is it hesitating on WOT between idle and 3k, or on gentle acceleration, or both?

 

If I stomp on the throttle it does better, so the acceleration pumps seem to be doing their job.

 

I will pull the emulsion tubes and compare them with the recommended ones.

 

Thanks for all the suggestions/Anker

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Given that, my first look would be at the idle jets. I'd pull them and blow them out first. What # idle jet are they? And what size choke do you have? I'd also inspect to see how the progression holes look as the butterflies open.

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One of my carbs will slobber some, I ran the car W/O the K&Ns and the clam on that side needed clean-up after a blat. That may have something to do with the amount of overlap between intake and exhaust. I've put the filters back on, but it did sound good unfiltered.

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Accel Automotive in Waltham, MA checked it today. Their assessment is that it runs too lean in the 2,000 to 3,000 range and needs bigger idle jets. They are on order now and I will take it back on Wednesday to have the bigger jets inserted and the car road tested. I hope that takes care of the issue.

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Look forward to the report. Changing jets is dead simple if its better but not perfect and you want to keep tweaking yourself. I'm experimenting with Keith Frank's emulsion tubes and various main fuel and air jets as well as idle jets (actually I'm on hold while I rebuild my starter), and happy to share bits once I get settled on the right mix for mine. I will say that his emulsion tubes are an improvement over the Weber F16s, at least on my setup.

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I got the car back today. They diagnosed the problem as lean running at light and mid-throttle because of undersized idle jets. They were right! It's like night and day. Spitting, sputtering and hesitating is gone. Idle isn't great, but I suspect that's because they didn't adjust the idle needles with the engine hot. It idled great when cold.

 

I am a very happy camper and I really like Accel Auto in Waltham, MA. They aren't cheap, but I don't care. I so all maintenance and repairs myself, and I am happy to pay a shot to deal with the difficult stuff!

 

The car has had the problem since new in 1987. I am surprised all previous owners, 3, haven't had it dealt with!

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  • 2 weeks later...

I had something interesting happen.

 

After the new idle jets were installed it runs much, much better. The smoking has also changed. At first it would smoke a lot less when hot, but today it barely smoked at all, even when cold. My guess is that a piston ring was stuck and now has become unstuck.

 

Interesting!

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