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  #11  
Old 4 Days Ago
ProLibertate ProLibertate is offline
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Regarding your response, Ferrous Head: the plugs and an overwhelming smell of gas are all I'm going off of here. My wife remarks every time I come in from working on the bike how much I reek of fuel.

The engine does stumble and the carb backfires when I do a quick blip-- rolling it on smoothly produces a steady increase in engine speed.

Last edited by ProLibertate; 4 Days Ago at 04:17..
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  #12  
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Ferrous Head Ferrous Head is offline
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Quote:
Originally Posted by kitabel View Post
Have you dismantled the choke plunger? It may not seat all the way.
This man has a lot of experience and I would listen carefully to anything he says.
If it's running way too rich but only at idle (the smooth roll on suggest that) then either their is a problem in the idle circuit or the choke/enrichner.

I say choe/enrichner as I am not 100% au fait with these carbs. But the "choke" circuits on carbs work one of two ways. They either restrict air intake (or add another air path) or they supply more fuel.

So if you can definitely rule out the entire idle circuit, jet and passages, then it has to be the choke/enrichner.

They have a complicated little setup on the enrichner on these. Spring loaded slide ? I would look carefully at that an make sure it is working correctly.

Never assume a new carburetor is OK. Amal produced a BIG batch of MKII concentrics that did not have a required air passage drilled into them. Some people spent a LOT of time diagnosing those carburetors before it became common knowledge.

If the PO thought the bike was "turn key" he might have been right at that time. Gas sitting in a carb eventually turns into a"varnish" like substance that will certainly gum up passages and jets etc.
And with today's blends of fuel, well ....... Who knows what's in there ?
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  #13  
Old 4 Days Ago
ProLibertate ProLibertate is offline
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I didn't mean to come across as impertinent; I just simply think we can rule out the enrichener plunger. It's really an incredibly simple design, and visual confirmation will tell you whether the plunger is fully seated or not.

A little back-story here... when I purchased the bike, it had an Andrews Flow Master (AR40) installed. It would bog and fall flat on its face at around 3,500 RPM, and I had tons of fuel just sitting in the throat of the carb. Needless to say, the plugs were terribly carbon-fouled then also. I figured it was probably a faulty accelerator pump, and since finding parts for that carb are exceedingly difficult, I decided to go the VM38-9 route, as they come highly recommended by many. Unfortunately, my problem with fouled plugs persists.

It's recently come to light that I shouldn't rely on plug chopping to indicate proper tune, but I have a hard time deviating from what has worked so well for me in the past. I've always aimed for caramel or light gray plugs, and I've never had an issue with performance or gas mileage. I'm having a hard time understanding why I'm not getting the results that so many others have right out of the box. The bike will start up on the second kick and seems to idle quite nicely, but the pitch-black plugs have me baffled.

I guess at this point I'll check the timing and points gap and will report back.

Here's a short video during idle: https://youtu.be/Cy8-P82O4xxxx/a>

Last edited by ProLibertate; 4 Days Ago at 04:20..
  #14  
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Quote:
It would bog and fall flat on its face at around 3,500 RPM, and I had tons of fuel just sitting in the throat of the carb.
A heavy mist of fuel you can see at the mouth of the carb is what we call "stand off" and can be an inevitable result of the cams and intake tuning.

I watched the video and this looks to be a very nice bike. Someone has spent serious money on it. And from the sound of the engine those aren't P cams in there.

I think you said (and looks like) the bike has stock mufflers on it. That's a good thing as I suspect that a set of drag pipes would make it unreadable on the street.

If the engine is still 900 cc's the 40mm carb would probably be just too big. In conjunction with big cams that had late closing intakes (to make it kick startable) the engine would be missing a fair bit of torque down low. When you hit the reversion point it would be like hitting a brick wall.

What I would do now.

Find out what cams are in there if you can. Ask the PO. He should be happy to tell you.

Do a compression check. That will tell you a lot. My guess is your going to be on the low side at cranking speeds.

The serious way to jet (tune) the carby is via an A/F sensor. On the dyno. This bike looks like it deserves it. I've been playing with motorcycle carbs for over 50 years. I have one of the best ears for engines at the track.
But I cannot match what an A/F sensor will tell me.

With the Mikuni you should be able to get a very flat line. Spend the time and effort and reap the rewards.

The crossover point for inter's and mains is very often a rich point on carbs. That's usually found somewhere around 3,000 RPM.

Oh. Stock jetting works very well in stock engines. Hot rodded engines, not so much.

Valve changes, port work and cam changes will all change what an engine needs in terms of carbonation and ignition timing.
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  #15  
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Sounds awfully restricted through ex note to me.
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  #16  
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I really appreciate you being so accessible, Ferrous Head.

It looks like I've got my work cut out for me! Fortunately, I've just been laid off for the season, so I've got a fair amount of free time on my hands.

The previous owner didn't really know much about the bike. Apparently when he got it, it was mostly put together, but he still had some finish work to do on it. Prior to his ownership, it supposedly belonged to a fella' that was really in to the dyno/drag racing scene; he passed away from cancer and his sister was tasked with selling it. When I purchased it, it hadn't even been transferred into the new seller's name... it still had the old title with POA. Obviously, this makes ascertaining just what components have been installed a difficult proposition.

I've got a folder that came with the purchase that has tons of loose-leaf papers with ratios and measurements on them. I don't know how long ago these were taken, but some of the info is as follows:

Bore: 3.236
Stroke: 3.8125

If this is true, it would've been bored to over 1,000ccs, right?

Front compression ratio is given as 9.778 and the rear is 9.581.

I'll do a compression check and see if these numbers still hold true.

Last edited by ProLibertate; 3 Days Ago at 01:52..
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  #17  
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I think you hit the jackpot !

Not stroked but bored. Well, not bored as you can't really go that far with stock cylinders. I suspect an examination of the cylinders will prove interesting as they are going to be aftermarket big bores.

And the fact that he is so precise with his numbers does indicate someone who built this engine had serious knowledge about what he was doing.

I now suspect you might find pretty respectable numbers on the cranking pressures. I still think it will have pretty "big" cams in there which will keep those numbers down but drag racers all build for torque.

I am guessing again now but suspect maybe the engine hasn't been broken in yet. If that's the case you need to do that carefully. Try to fins out if the PO ran the bike at all.
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The comp ratios he cites are more what I would expect to see on a en engine built for the street. But I would be looking for Avgas for the bike if I owned it.
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Really? You think so? Thanks for the lift. I was pretty deflated after discovering the bike's not "turn key" as was advertised; I traded a pretty nice '01 Softail for it.

Is there any way to determine the manufacturer of the heads externally? Any stamping or anything like that I should be looking for? I kind of hate to go tearing the bike apart, but I'm really curious about just what exactly it is I have here, and am hoping determining the modifications that have been done might help develop a baseline for jetting/tuning.

I believe the fella' that initially did the work on it passed away in the process of doing the engine rebuild, so I don't think it has been broken in. The gaskets appear fresh ad there's lots of untarnished chrome engine hardware. The guy I bought it from was a truck driver and it sounded like he was hardly ever home, so I don't think he had a chance to ride it much (if at all).

Assuming the bike has had the modifications done that these notes woukd suggest, is the VM38-9 a poor choice? Is it "enough" carb in your opinion?

I'm honestly not quite sure how I feel about the mods that have been done. I know a lot of guys would kill for a lean, mean, drag racing machine, but I'm not a speed demon-- I was really just looking for something to put around on the weekends and go to vintage bike meets. I mean... aviation gas? Seriously?

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Unless you are prepared to take it apart.
Just fire it up and give it what it wants.

A VM38 ought to be good for over 80 HP
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