hesitation at cruising speed

[j.hall]

I'm experiencing a slight bucking/hesitation at around65/75 mph. Recently tuned so thats OK. Carburetor???

[WFBowen]

Does this happen at constant throttle or when you start to accelerate at that speed?
Bill.

[j.hall]

It happens most when I start to accelerate. I just read an interesting article in the Sept '03 issue of Excellence mag. under Tech Talk. It talked about getting the correct temperature spark plugs (the person who wrote in was having a similar problem). I'll start there but would appreciate your thoughts also.
joe

[WFBowen]

Could be the plug heat range. To be safe, I'd check the float levels and the accelerator pump action. Injection quantity is important, but exactly when they squirt (i.e. just when the throttle opens) is critical - especially in a taller gear where it's most noticeable. Double checking the sync of the carbs is also something to consider - I've seen a lot a cars (Solex & Weber) that are perfectly synced at idle, but at half throttle are way off - worn ball fittings, bent arms, incorrect pivot geometry, etc.
Hope this helps.
Bill.

[j.hall]

On closer inspection I noticed some slight staining (light brown to tannish if that makes sense). on #4 cylinders intake manifold.....appears to be dried gas?? I have noticed the smell of gas in the cab on occasion........not a lot but noticable.
The article I mentioned said the problem (with incorrect heat range plugs) would show itself at higher rpm's and would begin to be more noticable (as the plug wore) at lower and lower rpm's. I will install a new set......what are your thoughts on plugs?
I consider myself a fair mechanic, is carb. adjustment something left to the pro's??

[djdsc]

J

There have been extensive posts over the last two months over on the 912bbs regarding carb tuning and balancing. Some very fact based testing and discussion on the subject.

Excellent postings on Solex, Weber's and now a Dell discussion.


http://www.912bbs.org/

[j.hall]

thanks.....I'll check it out.
joe

[WFBowen]

Joe:
If you're dead sure your ignition (especially the maximum timing) and valves are adjusted properly, then I'd look at the carbs. Syncing is not a big deal if you have a metering tool to register difference in the 4 throats. If you have a "real" Porsche mechanic in your area, I'd suggest letting him take a look. There's a strict procedure to doing this and if done wrong, you'll be chasing the problem. Also, if the carbs/linkage are just plain worn out, it'll be just about impossible to get a nice running motor throughout the rev range.
IMO, the key to tuning these cars is setting them up for the higher rev ranges. Everyone (myself included) loves a smooth idling motor, but it's critical in an aircooled motor that the timing and carb mixture/sync is dead on at higher revs. When they were brand new, this wasn't a problem. As things slowly change dimensions, sometimes it gets into a trade-off situation.
As for the plugs, I've used NGK's with no problems. I don't know how far you live from a fwy/hwy, but if you can, run the car for a short trip at around 55mph sustained, then "limp" it home and pull the plugs. Reading them will give you a good idea of what's going on & if there's a possibility that you might be damaging something at higher speeds.

[j.hall]

Bill,
Thank for your reply. I double checked the timing and installed new (ironically) NGK's yesterday and it made all the difference. I went for a sustained cruise of 65 for about 45 mins and had no hesitation. Near the end of the sustained 65 speed I eased it up 5mph at a time and held it for about 5 mins. the final cruising speed was 80mph. It showed no hesitation. I've always tried to eliminate one problem at a time so as not to compound an issue.
The carbs do need adjusting. And unfortunatly there is not a REAL Porsche mechanic close by. I will check the linkage today and order the proper metering tool. In Duane Spencers book he has a fairly detailed description of the balancing process. I'm going to attempt it. Have you done this before?
Thanks again, joe

[WFBowen]

Done it lots of times....as long as Spencer's method if followed, you should have good results. A few side notes: As I mentioned, in the real world, some parts will be worn. If you check the midrange rpm sync, try to do it by pushing down the crossrod arms, instead of adjusting up one idle stop screw .... this accounts for sloppy ball socket linkage. Also, there'll usually be some difference between front/rear throats on a used motor. I've seen some top Porsche mechanics "twist the throttle shaft" by using end wrenches on front/rear of the shaft and twisting it to equalize the throttle settings (pre 68 Solex). This was done on a running engine with a sync meter on each throat. Needless to say, I never had the balls to do this as it seemed to be one of those Saturday "tweak it a little more" projects that could easily end in disaster.
And.. I've had good luck with the inexpensive plastic sync gauges that are sold by VW suppliers... the old Unisyn tends to choke the motor when installed.

Also glad to hear the engine is smoother now - nothin like a fresh set of NGK's!
Good Luck,
Bill.