View Full Version : Bucking on Coast Down ?

11-29-2006, 11:59 AM
When I let off of the gas on my 3-spd/OD '50 Starlight it does not coast down smoothly. It bucks. If I tickle the gas it gets better but it is still there. Someone suggested that I replace the carbs primary jets (2ea 1bbl Hollys on my Cathcart 185). I went a bit larger, no change. Any help on this would be appreciated.

Harv [8D]


11-29-2006, 02:05 PM
You might want to check that the idle speed is not to high. You also might want to call Bill Cathcart and see what jets he reccomends. I would also check the float levels.


11-29-2006, 04:12 PM
Have the very same problem with mine, but it is much better now. My solution was the rear motor mount (Rubber)attached to the engine bell housing and the drive shaft mounts on either side of the drive shaft center bearing. Still have a trace of the problem.

Buddy...'54 Champion 2dr
Warner Robins, GA 31088

11-29-2006, 09:13 PM
A sticky timing advance could also cause this problem. I'd make sure the advance mechanism in the distributor is free and that the timing advacnes and retards with changing rpm. Use a timing light if you've got one, and if it's sticking you'll be able to see it.

Mike with Speedster

11-30-2006, 07:08 AM
According to my shop manual you should put a .020 feeler gauge between the distributer and vacuem advance at the base of the dist. before you tighten the clamp.


11-30-2006, 10:51 AM
I had the same problem with my 49 Commander, I checked everything and found that that it was a two fold problem, First if you read the manual it tells you to set the basic timing at the indicated mark with the fine adjustment set in the middle then advance the timing until a slight ping is noted on full load full throttle then back off until the ping just disappears. Problem with this method is it was based on regular grade gasoline in the 40ís, which at best was 75 octane. With today's 87 octane you can't get it to ping even if set on the full advance on the fine adjust setting. The fine setting I am talking about is the little tab that is held in place with the small cap screw. Well, full advance gives you great performance but you get a buck on deceleration. So it I set it back to the nominal setting but still had some buck, next I checked the vacuum at idle and found that if you set it as I was taught which is set the throttle plate screw to achieve maximum sustainable vacuum then adjust the mixture screw in (leaner) until the idle begins to get rough, then back the screw out until the idle smoothes out. Well here again this was with the higher hydrocarbon fuel we had in the 40-70ís. If you follow this today you end up with an excessively lean mixture when you are in a deceleration mode. So I raised the idle speed to about 600-700 RPM and set the mixture by going to the rich side until the engine missed then leaned it up to get the vacuum gage to stabilize. This has minimized the bucking to a tolerable level. I hope to get a CO meter and check the overall mixture while driving. According to the Air Cooled Volkswagen people (Yes I have a Beetle as well which had the same problem) todayís gasoline contains so much more oxygen that all old carburetors really need re-jetted to richen the mixture. Most old cars if set up to original spec's end up running too lean at highway speeds which causes valve damage overtime.

Gee the world gets more complex everyday!!!

Restore it, don't replace it.Keep the Studebaker reproduction industry going

12-01-2006, 07:56 AM
Thanks for the feedback Gents. When I got the engine from Cathcart in '02, the lg Holly 1-bbl carbs had two different size jets in them. One had a #52 & the other a #56. Bill suggested, at that time, to go a bit larger so I put in #58s with no real change noticed. Buddy mentioned that the carb on his std 170 engine has a #94 jet. I ck'd the Holly site about gen'l info on jetting. They give some detail on changing jet size and say that manifold vacuum & spark plug coloring (lean or rich burn) is what to watch for. Also that having to move up or down 8-10 jet sizes may be due to a carb not properly matched for the application. Still considering this issue.

I will definitely look at trying the great suggestions from you all and especially 1949commander's detailed help points.

Thanks again and Happy Holidays to you all, Harv [8D]