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Re: Can't find vaccum leak. Backfiring, missing on accelerat

Posted: Sat Dec 23, 2017 8:59 am
by McCall Pinz
Does this magically fix itself for a while and return in a few months? If yes, I'm chasing the same thing. Ran horribly last fall, ran horribly this spring, drastically improved one day and was flawless all summer. Now it's back again... dropping the fuel tank this weekend to see what lurks there.

Re: Can't find vaccum leak. Backfiring, missing on accelerat

Posted: Mon Dec 25, 2017 4:57 am
by one2many
boeing7837, earlier you said that one carbs acc pump was pumping half the volume of the other...what process did you use to calibrate the acc pump volumes?

Re: Can't find vaccum leak. Backfiring, missing on accelerat

Posted: Wed Dec 27, 2017 11:01 am
by boeing7873
Answering the previous 2 posts: not sure in what part of the precess I mentioned the accelerator pumps not working correctly, but I had definitely a problem in the very beginning when I rebuilt the carbs. On one of them I made a horrible mistake and put a plug where the pump pressure valve was suppose to go ( and the valve as a bowl plug) Of course it did not work. After fixing this I started to see that some nozzles deliver a clean solid jet of fuel, others deliver a spray or fan shaped flow. I may have believed the volumes injected were different. But I decided correctly after checking the rod lenghts and pumps that the problem was somewhere else.

Regarding the inconsistency of the problem, I am starting to believe it is a temperature/humidity related issue. My problem was that at low/mid range rpms, the mixture would be too lean.
I changed idle fuel jet from 55 to 60 and idle air jet from 130 to 110. Much much better now. We'll see with the temperature how it behaves in the future.

Re: Can't find vaccum leak. Backfiring, missing on accelerat

Posted: Tue Jan 09, 2018 9:17 am
by Hotzenplotz
one2many wrote:boeing7837, earlier you said that one carbs acc pump was pumping half the volume of the other...what process did you use to calibrate the acc pump volumes?
you can easily measure the amount.
remove the carb and attach a fuel hose to a bottle hung a little higher , in order to deliver fuel by gravitation
set the carb on top of a glass and operate the fuel pump several times. divide by the number of strokes you did, and you get the volume per stroke. that should be about 1,5 cm³

Re: Can't find vaccum leak. Backfiring, missing on accelerat

Posted: Tue Jan 09, 2018 6:00 pm
by one2many
Thanks Hostenplotz, Yes there are multiple techniques for calibrating pump volume, from how the OP described his problem and the steps he took to rectify it I was thinking that he/she might have missed this step when working on the carbs.
I recently posted a link to "sparking plugs" website, It's Porsche specific (32 NDIX) but it has heaps of relevant to us info regarding the 36 NDIX including a great step by step for calibrating pump volumes...something our otherwise pretty thorough repair manuals doesn't even touch upon.

Considering having calibrated pump volumes is crucial to the proper running of any multiple carb system and the fact that the Steyr manuals don't seem to include the steps I must admit I have often wondered how many air cooled Pinz owners have bothered with this step.

I don't know about the workshop standards around the world but here in Australia they're MOSTLY (not all) hugely overpaid and grossly under skilled/trained or more likely just thieving money lovers either way if you wanted a high QUALITY carburettor repair the last place you'd take them would be to one of the remaining carb repair businesses left in business. Sadly it seems, like good honest skilled labour all the carb shops went out of business in the late 80's. Experience has taught me the only way to be sure these type of jobs are done properly and with a level of care you would expect is to do them yourself....besides that's half the point of Pinz ownership isn't it? To appreciate these awesome machines both in performance and mechanically, that is to appreciate first hand some of the best automotive engineering from its time.
Have fun, Get filthy.

Re: Can't find vaccum leak. Backfiring, missing on accelerat

Posted: Tue Jan 09, 2018 7:18 pm
by Jimm391730
if you wanted a high QUALITY carburettor repair the last place you'd take them would be to one of the remaining carb repair businesses left in business.
There has not been a carburated auto sold in the USA since 1994 (24 years ago) so almost no one today has any significant experience with them. For our Pinzgauers, Jim L. is one of the best at rebuilding and setting the carbs. If in doubt, send them to him for an overhaul. It is well worth it.

Re: Can't find vaccum leak. Backfiring, missing on accelerat

Posted: Tue Jan 09, 2018 9:04 pm
by one2many
For those more interested in doing the above mentioned job (pump/piston fuel volume correction & calibration) themselves, for systems that use more than one carb you will need to use a method that measures against a unit of time as well as measuring the fuel per amount of strokes...that is if you want to include the "dampening spring" for quick pedal depressions at the bottom of the piston and to a lesser extent the pressure valve (fault finding only) in your results, well worth it since you've gone to the trouble anyway.

Re: Can't find vaccum leak. Backfiring, missing on accelerat

Posted: Sun Dec 01, 2019 8:46 pm
by boeing7873
One more iteration on my search for the solution to this problem... I have smoked the carbs, installed, and found leaks in the cold start plate covers, they were bent. Also, my carb springs... one broken, one missing altogether!!!


boeing7873 wrote:
Wed Dec 27, 2017 11:01 am
Answering the previous 2 posts: not sure in what part of the precess I mentioned the accelerator pumps not working correctly, but I had definitely a problem in the very beginning when I rebuilt the carbs. On one of them I made a horrible mistake and put a plug where the pump pressure valve was suppose to go ( and the valve as a bowl plug) Of course it did not work. After fixing this I started to see that some nozzles deliver a clean solid jet of fuel, others deliver a spray or fan shaped flow. I may have believed the volumes injected were different. But I decided correctly after checking the rod lenghts and pumps that the problem was somewhere else.

Regarding the inconsistency of the problem, I am starting to believe it is a temperature/humidity related issue. My problem was that at low/mid range rpms, the mixture would be too lean.
I changed idle fuel jet from 55 to 60 and idle air jet from 130 to 110. Much much better now. We'll see with the temperature how it behaves in the future.