07-30-2017, 09:09 PM
Need help with pop off specs for a Mac flat back carb on alky.
Thx. Dean
Thx. Dean
Carb pop off specs
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07-30-2017, 09:09 PM
Need help with pop off specs for a Mac flat back carb on alky.
Thx. Dean
07-30-2017, 10:31 PM
pretty sure SteveO would advise 20# so the duck bill will retain function.
that's if you're running the triple plate version of the bdc's.
Dave L.
07-30-2017, 11:39 PM
Dean, by "flatback" you were referring to the small Mac carb (Mc9 or earlier), and not a BDC-X (Mc91 and up), correct?
The duckbill circuit started with the BDC's, I believe. I don't think the small Mac carburetors had the duckbill circuit. If that's true, then it seems like the desired pop-off pressure should be close to the value folks use for Tillotson HL's.
07-31-2017, 08:29 AM
doug t. is correct
i was tired last night. or getting dumber your choice. blueprinting the carb before you do anything would be helpful in determining pop off pressure. but for purposes of generally answering the question flatbacks on alky will like around 7.5-9# pop off . this is a good general area to start. that's not written in stone , there's alot of variables in every carb adjustment but if the carb was built/modified correctly that should be the area you'd want to start at to get running.. carburator performance is controlled by all sorts of variables i've listed a few below plus there are things that you can't control like the weather and fuel conditions. So it is useful to know exactly what is done to your carb. things like the inlet needle passage size , the passage size for the control needles , are the control needles modified (ie shortened or thinned) the inlet holes in the idle circuit under the welch plug, and the size of the inlet holes leading to both the idle and high circuits in the fuel reservoir are all part of deciding pop off pressure. the fulcrum lever height adjustment also needs to be taken into account. this is called blueprinting and is extremely useful in assuring good track results and carburators NOT being throw into the woods in frustration!! this is a long drawn out process to determine all these factors. far to long to post here but if you are interested in that sort of thing i'll be updating the carb post to attempt a step by step blueprinting instructional. anyone seeing mistakes in my instructions should feel free to correct me i am not perfect!! go here to read further https://www.vintagekartforum.com/newreply.php?tid=1636 Save Save
Dave L.
08-02-2017, 07:10 AM
(07-31-2017, 08:29 AM)David Luciani Wrote: doug t. is correct
08-02-2017, 07:28 AM
(This post was last modified: 08-02-2017, 07:40 AM by doug tenney.)
Dean, I might have you covered on this one. See attached PDF.
If it weren't for Steve O's exquisite treatises on BDC's, I would've switched over to a pair of Tillotsons years ago. (Just think of all of the tools that would've become rusty in my toolbox by now... not to mention the added wear and tear on my karts from more time on the track... but I refuse to give up... lol)
08-02-2017, 09:18 AM
any information that was found on mcbob's website and copied would be greatly appreciated if it was posted to this forum. i heard that mcbob passed away last weekend july 22nd. and i'm repeating this with great sadness. i had emailed him after hearing this but hadn't gotten any response. i didn't want to start any rumors. i had heard this from a friend of his in australia. in the past he had always helped me with any questions i had pertaining to mcculloch saw motors and mcculloch saw parts. he was always happy to share his knowledge. the information once found on his website was extremely helpful. not into sharing bad news but i felt this had to be shared. he was a great asset to both the vintage karting world and the vintage saw world.
08-03-2017, 08:58 AM
Sorry to hear about McBob. Thanks, Chuck, for passing that on.
Being the paranoid-hoarder type, I saved a lot of kart-related files from McBob's site and the forum over the years. (Let's face it, most of us are a little "off," and we come to the kart forum to feel "normal!!") Of course, I only saved the files that pertained to stuff I was working on. Even with that, it would take many hours (days?) to re-post all of it... it's stored in various directories, etc. As in this case, if someone describes something they saw, and I have a copy, I will gladly share it (ahem, provided that it is not copyrighted). |
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