371bb

[parrisw]

Your getting into very dangerous territory Terry. This class husky is very addictive!

Boring that carb wasn't without incident. It ####ed with my mind for weeks till I worked out the issue I was having with it. At home it ran like #### of a shovel. I'd take it out bush with approx another 3-400 metres above sea level and it would play up like you would believe! I eventually pressurised it under water and bingo! I'd popped through to one of the primary ports! Man does nail polish work well for sealing little carb sealing issues!

Here's the saw running still with issue's prior to find the problem.

The carb sits on the shelf these days as I've been running a different kit on it lately.

I had difficulty finding an inlet manifold to match the increase in size. I was going to manufacture one but never got there. I was going to play with runner length to match inlet pulses but didn't get around to it. Maybe in the near future. I'm bidding on a lathe so that will renew the interest and apart from some cyl work, making some inlet pits and pieces may prove interesting.

Here's the saw running in a firewood environment (hardwood) with the carb fixed. nothing else done.



Sorry Will, let er rip rip rip!

No need to be sorry mate. Anything add to the thread.

Do you feel that it would help to just bore out the venturi? and leave the inlet and outlet of the carb the same?

 

[AUSSIE1]

No mud, I had to make up some clay out of flour, water and vegetable oil. I tried some children's Play Dough, but the stuff was too elastic. The clay I made up was a lot firmer, it would hold the shape of the port and was able to be removed without stretching. - After I finished with it, I gave it to the chickens and they thought it was Christmas.

I don't know what the bore size is on the 19mm Walbro WJ carbs (the series on the 385). If it is the same as the Zama C3M EL8 carb, then there may be room for some more work on the venturi.

The reason I'm looking at the carb options is that I would prefer a shorter intake duration with a bigger carb versus a smaller carb and a longer intake duration. I'm doing a bit of research before I start hacking away at the BB jug. I'd hate to find out later that there was a bigger carb for the engine.

Just to put this carb size issue in perspective, I'm running the equivalent of a 19mm carb on my 50cc 450, so with that background I'm not feeling all that comfortable with a 17-17.5mm carb on a 77cc engine.

When I get a chance I'll get the measurements off my 385 carb for ya. Like Boleclimber I looked at the larger saw carbs and apart from the tilt there is room issues if you want to keep the housings stock. If I get the lathe I will be able to play a little more with the manifold side of things.

Do you feel that it would help to just bore out the venturi? and leave the inlet and outlet of the carb the same?

Gets a bit more involved than that Will.

 

[Boleclimber]

The bore size at the throttle side of both a 372 and 385 carb is .810 inches. The choke side a 372 carb I have measures .878 inches, the 385 carb measures .882 inches. The 372 carb is 1.332 inches long and the 385 carb is 1.512 inches in length.

 

[Terry Syd]

Hmm, so the bore size is 20.5mm (metric in Oz) and with a 17.5mm venturi that only leaves 1.5mm of venturi material around the bore to work with. That's not much of a venturi to start with, a bit more taken out and I might as well run a straight pipe.

I'll check with Zama tomorrow and see what they may recommend. Perhaps they have a 17.5mm carb that was built with a larger bore.

EDIT: I just noticed that Zama listed a different throttle plate number for the 17.5mm carb series. Those carbs may have a slightly larger throttle diameter.

I'll let you guys know what I find out.

 

[AUSSIE1]

Hmm, so the bore size is 20.5mm (metric in Oz) and with a 17.5mm venturi that only leaves 1.5mm of venturi material around the bore to work with. That's not much of a venturi to start with, a bit more taken out and I might as well run a straight pipe.

I'll check with Zama tomorrow and see what they may recommend. Perhaps they have a 17.5mm carb that was built with a larger bore.

EDIT: I just noticed that Zama listed a different throttle plate number for the 17.5mm carb series. Those carbs may have a slightly larger throttle diameter.

I'll let you guys know what I find out.

Thanks Boleclimber.

Thanks Terry. Sorry I had forgot to mention that it was evident by eye the venturi's were of different dia but I hadn't measured them.

 

[Terry Syd]

It's been a couple of days and still nothing back from either Zama or D&G.

A couple of other things to note about the C3M FJ series is not only is the venturi larger at 17.5mm, but it also has a different main nozzle. It could be that the FJ series uses a shorter nozzle for the larger venturi.

The fastest airflow is along the sides of the venturi. As the airflow increases in a carb, the airflow in the center of the carb increases. A longer main nozzle will 'lean' the mid-range of the carb as a result of those differences in airflow. The areo builders have been using slightly longer main nozzles to lean their midrange.

On a saw, it often seems as if the carb leans out as the revs drop. Probably most of this is related to high POPs, but a longer main nozzle would also contribute to the leaning affect.

That is something I noticed when I was boring my first carb, the nozzle stuck out further into the venturi. It's true that main nozzles can be inserted differently to change the nozzle length in the venturi, but starting with a short nozzle for boring a carb would be the proper way to go.

I also noticed the price differences at D&G for the C3M FJ series. The FJ2 is only $68.73, but the FJ3 and up are $265. C3M-FJ2 - C3MFJ2/C3M-FJ2 ZAMA CARBURETOR PART C3MFJ2/C3M-FJ2 - Replaced by: C3MFJ2A/C3M-FJ2A Carburetor - ZA-C3M-FJ2 - ZAMA CARBURETORS

Bugger if I know the difference in the carbs. Since it is really just the body of the carb that I would need (throttle shafts etc come off the original carb), the cheaper FJ2 would be the way to go.

I'm still trying to get more information about the FJ series, but at this stage there are too many unknowns for me. I have a non-Zama carb on the saw and I don't know the compatibility of the shafts with a Zama carb. The postage to Oz would also add to the equation.

Anyway, I thought I would throw this information out to the forum in case anyone else building a 372 might find it worthwhile.

 

[AUSSIE1]

Thanks Terry, keep us posted.

 

[Terry Syd]

D&G replied, they are distributors and directed me back to Zama.

I found the application chart for Zama carbs. The C3M FJ carbs were listed under Fuji, for EC-10 and 12 engines. After doing more research, the only picture I could find for the carbs showed them with a float bowl. If that is the carb, it isn't even close to being a compatable swap.

I suspect that Zama came up with the C3M FJ carb as a swap for the float bowl Walbro. It might even have been an upgrade as the engines were used on 'stompers' for packing dirt - not exactly the best environment for a float bowl carb. The engines were 100cc and ran at 3,600 rpm.

Back to Zama for some info.

 

[parrisw]

Thanks Terry, your definitely thinking more into this then I ever do. Good reading though.

 

[Terry Syd]

Yeah Will, I'm an obsessive tuner. The wife gets irritated with me modifying everything I get my hands on - "can't you just leave things alone!"

The 372BB has some potential. I should be able to get all the flow issues resolved, except for the damn carb. It seems like a shame to have just one bottleneck that I can't get around.

 

[AUSSIE1]

Yeah Will, I'm an obsessive tuner. The wife gets irritated with me modifying everything I get my hands on - "can't you just leave things alone!"

The 372BB has some potential. I should be able to get all the flow issues resolved, except for the damn carb. It seems like a shame to have just one bottleneck that I can't get around.

Maybe you could pose the question over yonda. That's where the carb swapping etc usually takes place.

I started looking into it but put it on hold for the time being. I was going to bother some for some specs on some carbs from maybe the Stihl range. If I had to punch out the back of the saw to fit the associated parts, so be it.

 

[parrisw]

Yeah Will, I'm an obsessive tuner. The wife gets irritated with me modifying everything I get my hands on - "can't you just leave things alone!"

The 372BB has some potential. I should be able to get all the flow issues resolved, except for the damn carb. It seems like a shame to have just one bottleneck that I can't get around.

Ya. I'm definitely interested in it, if I could get a good gain from doing some carb work, I'd love to, then the bottle neck would be the inlet elbow after the air filter, I was thinking something could be fabbed up out of aluminum. The person to talk to about boring carbs out would be Brian aka Timberwolf, he gave me some good pointers a while back when I was going to do it, then I never got around to it and have since forgotten, haven't seen him around in a long time.

 

[Terry Syd]

Boleclimber mentioned he used a carb off of the 385 on his alky 372. That Walbro WJ series will go up to 19mm venturi size and still have the same distance between the mounting bolts as the 365/372 carb, unfortunately the carb is tilted on the bolts and the carb is a bit longer.

Since the Walbro WJ series can get a 19mm with the same bolt pattern, it would seem that there might be a way to get the same venturi/bore in the original carb. I was hoping that the Zama C3M-FJ series may have the potential to get it close to 19mm with a minimum of work.

Sure would like to get some info on that FJ series.

 

[Terry Syd]

Perhaps if Zama got a few inquiries from different people about the C3M FJ series carb they might reply. Here's their email address if anyone is interested - [email protected]

I was interested in the bore size of that carb as we already know the venturi size, 17.5mm.

They might even have some suggestions if they knew it was going to go on a chainsaw.

 

[MCW]

Maybe a 45mm DCOE Weber carb Terry?

 

[Terry Syd]

One of the posters on 'the other site' welded up a mount on his 372 jug so he could run a 22.2mm venturi HL 360. Now that I could get into, although I would be happy enough to get 19mm.

I sent Zama another request for information. If the buggers don't reply I might just go ahead and order the carb and maybe I will be able to adapt it. If I can't, I'll have to find a 'dirt stomper' that needs a spare carb.

 

[Boleclimber]

If the stomper carb is a float bowl design as you mentioned earlier, I think you would have running issues when tipping the saw on its side or upside down. Bowl carbs usually are gravity fed and in some instances auxiliary pump fed. With a chainsaw fuel tank low, a auxiliary pump would need to be added.

 

[Terry Syd]

Unfortunately, I don't know what the carb looks like. What I really wanted was the carb body. It appears to be based on the C3M body, so if it was a float bowl carb, I should be able to convert it by just bolting on the standard pump used on chainsaws.

Even if I couldn't enlarge the venturi any further, it would at least be 17.5mm. As for the bore size, the different throttle plate may be related to a different cutaway on the plate, but who knows at this point.

If it did have, say a 21-22mm bore, then taking it out to 19mm wouldn't be a problem. In fact, because of the larger displacement of the BB kit, there should be plenty of depression/signal at the venturi. I wouldn't be worried about getting it to meter fuel properly, I could always tweak the POP and throttle plate to dial it in for the BB.

 

[Boleclimber]

I didn't realize the basic layout was similar. This will be interesting if you can modify the carb to a pumper.

 

[Terry Syd]

I really don't think it will be a problem. If you take a look at the listing for parts for the carb you will see the pump gasket (#16) is the same for all the other variations. It appears the pump itself is a different configuration, but bolts on the same carb body. I can just swap the regular pumper system on.

Heck for all I know, the FJ2 pump section could be a high performance upgrade I'd want to keep!

Here's the listing for the C3M carbs. Go down to the C3M FJ2 listing and you can see all the common parts with the other carbs (metering springs, lever, screws, etc)

http://www.m-and-d.com/pdfs/zama/C3A_M_PARTS_LIST.pdf

If this works out, I'd bet they'll be a run on these carbs from the modders on this forum.