Anyone have port timing #'s for NWP Makita/Dolmar 84cc P&C ?

[Terry Syd]

I don't know how the base compressions were measured, but those figures seem high to me - especially since this engine isn't using a crank with stuffers (and the transfers extend all the way into the cases). Perhaps it is the formula that is used, but I wouldn't figure the stock 64cc engine having a base compression as high as 1.62.

The 1mm increase in the venturi of the carb may help, but I'd expect that a new carb would be a better approach. Say the intake manifold had a 20mm diameter, I'd be looking for a carb with something like a 19mm venturi and then blend the throttle chamber of the carb into the intake manifold. The smaller the throttle valve the better.

Perhaps someone has used a bigger carb on the Dolmar and can chime in with what is available.

 

[parrisw]

What saw was designed first? The 65cc or the 79cc? Wonder if they made it for one or the other first?

I always thought tight bases liked more blowdown? am I correct? I'm running 30°blowdown on the BB, and it runs great.

 

[madhatte]

This is an excellent thread. I am big on graphs, and this is a reminder of why.

 

[tdi-rick]

[snip]

The 1mm increase in the venturi of the carb may help, but I'd expect that a new carb would be a better approach. Say the intake manifold had a 20mm diameter, I'd be looking for a carb with something like a 19mm venturi and then blend the throttle chamber of the carb into the intake manifold. The smaller the throttle valve the better.

Perhaps someone has used a bigger carb on the Dolmar and can chime in with what is available.

The smaller the butterfly the better Terry ?

I'm planning on going to a slightly bigger throttle bore/butterfly and slightly larger venturi (only 0.5mm, although I could bore it out a touch more) to try to retain torque as it will maintain depression across the main jet yet flow a little better than the stock carby.

There's another version of the C3M carby off a Fuji-Robin 115cc saw that I'm trying to track down (they are stupidly priced new, over US$200) so i can measure it and either modify and fit or modify a 7900 carb with a new throttle shaft/plate.
The Robin engine only spins to 6,000 RPM or so but hopefully it has a bigger bore, it definitely has a bigger venturi (17.5mm vs 17mm) and the throttle plate, shaft and choke assembly are totally different part #'s to all the other versions of the C3M carby.
The Robin carby is either a C3M-FJ3/FJ4/FJ5.

This should (in theory ) be an easier fit than a Tillo. maybe.

I wish the old injector Tillo's were made a bit bigger than for a 68cc saw, it'd do away with the restriction of the choke, particularly the way the inlet is configured in a 64/73/7900.

 

[Terry Syd]

If you define 'tight bases' as engines with a higher crankcase compression, then the tighter the base (higher the compression) the less effect the blowdown pressure has on the flow through the transfers.

With less pressure in the crankcase, then the more the pressure of the residual exhaust gases at transfer opening can reverse the flow in the transfer ports. Further, after the transfer flow is finally established, the pressure in the crankcase then has to make up for lost time. A higher crankcase pressure will help to make up for that lost time. Essentially, you can get away with a shorter blowdown time/area with the higher crankcase compression.

One thing that concerns me is whether these BBs are designed to be run with a stock carb. If so, then the intake timing may have been lengthened to compensate for the smaller carb.

Is the 6401 carb the same size as the 7901 carb?

 

[Terry Syd]

Rick, check this webpage for the C3M -

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

Edit: Here's the webpage for the site - http://www.m-and-d.com/zama_carburetor.html

Their prices are very reasonable, even if you have to get it imported to Oz.

Yeah, a smaller butterfly (or throttle bore) - in the example I used of a 19mm venturi carb flowing into a 20mm intake manifold, the throttle bore could be any number of sizes. If the throttle bore was 22mm, then you would only have to clean up 1mm around the entrance of the intake manifold to adapt the carb. If it was 24mm, then you would have 2mm of material to blend in.

 

[mtngun]

Is the 6401 carb the same size as the 7901 carb?

I believe so.

Sorry, I didn't get to the hypothetical bigger carb today. Instead, I let the laptop chug away at some different MOTA settings that T-Dubya had suggested. Will try again tomorrow.

Bell's method to measure base compression has you remove the carb, spin the piston to TDC, point the saw down, and pour your measuring fluid through the intake port until the fluid has filled the jug. That's your crankcase volume (CCV).

Crankcase compression = CCV / (CCV - CV)

Where CV is the displacement.

On the 64cc jug, CCV was 167 cc, so that works out to 1.62 case compression.

I prefer Bell's method because it's simple.

MOTA has a more complicated and potentially less accurate procedure for measuring base compression. MOTA insists that you collect the data MOTA's way and let it calculate the case compression compression for you. :msp_sad:

If I ever reach the point where I think I know what I'm doing , I'll post a thread showing how to do all the MOTA measurements.

 

[Terry Syd]

Hmm, just doing some rough calculations - if the carb has a 17.5mm venturi, then the area of the venturi (disregarding the obstruction of the main discharge nozzle) is 240 square millimeters.

A 346NE has a 15 mm carb - which has an area of 176 square millimeters.

The 84cc engine is 68% larger than the 50cc 346NE, yet the carb is only 36% larger than the 346's carb.

In order to match the relative areas of flow of the 346 carb, the 84cc jug would need a carb venturi of 19.4mm.

I feel almost guilty that the area on my 450 Husky (50cc) is a 13.5mm carb, plus another 13.5mm inlet on my strato port - the total is 286 square millimeters - or, 19% larger area than the carb on the Dolmar. - Which helps to explain why I can get by with only 154 degrees of intake duration.

 

[tdi-rick]

Cheers Terry, I already have the parts lists, tech docs and whatever else i can scrounge for Zama, Walbro and Tillo, but that link will help a few people I'm sure.

Their prices are very good too, thanks again.
I need a spare carb to tinker with.

Yes, the same carby is used on the 64cc, 73cc and 79cc versions.

 

[tdi-rick]

It's actually a nominal 17mm, which = 227mm2

 

[parrisw]

Word on the street is a Tilly HS 265A will fit with a little work, don't know the particulars.

 

[tdi-rick]

So I've read Will, and the information is out there

mtngun, what rod length do you have for it ?
I found it bloody difficult to measure and only have an approx. length of around 66mm

 

[parrisw]

So I've read Will, and the information is out there

Any idea on the advantages? I'm guessing bigger bore?

I was thinking of trying to bore the stock carb. Have never attempted that before.

 

[AUSSIE1]

The 372, 385 and 395 run the same throttle bore size.

The Walbro carb of the 372 I opened up the throttle side 1mm and fitted the choke butterfly. I was able to open up the shaft quite easily. For the choke butterfly I had a washer with the correct thickness and the screw hole the correct size. What a fluke!

The 365BB running this carb. Needs a better chain than the round filed chisel I know!

<iframe title="YouTube video player" width="640" height="390" src="http://www.youtube.com/embed/C2gqY5hZUGY" frameborder="0" allowfullscreen></iframe>

 

[tdi-rick]

Slightly bigger throttle and choke bores Will so in theory will pass more air but in practice it all comes down to the porting and where your power band needs to be.

Done well young Al.
I reckon there's plenty of scope in the stock carbies for a modded work saw.
The transitions are always rough so can be smoothed/reshaped and polished, throttle and choke shafts can be thinned, screw shanks ground right down and loctited in (222 always worked well for me, I've never lost a butterfly in God knows how many practice and racing miles) and heads thinned and butterflies thinned on the leading and trailing edges.
Hell, I even used to align the screw head slot up with the air flow.

 

[AUSSIE1]

Slightly bigger throttle and choke bores Will so in theory will pass more air but in practice it all comes down to the porting and where your power band needs to be.

Done well young Al.
I reckon there's plenty of scope in the stock carbies for a modded work saw.
The transitions are always rough so can be smoothed/reshaped and polished, throttle and choke shafts can be thinned, screw shanks ground right down and loctited in (222 always worked well for me, I've never lost a butterfly in God knows how many practice and racing miles) and heads thinned and butterflies thinned on the leading and trailing edges.
Hell, I even used to align the screw head slot up with the air flow.

Yes, there is alot of cleanup that can be performed on the stock carb for sure and for a work saw and in particular the 371/2 this will suffice.

 

[mtngun]

mtngun, what rod length do you have for it ?
I found it bloody difficult to measure and only have an approx. length of around 66mm

I also came up with 66mm, but as you said, it's tough to measure and could easily be off a mm or two. Fortunately, small deviations in the rod length seem to have little impact on the results.

As requested, here's MOTA's take on a hypothetically bigger carb venturi. I went from a 16.5mm (someone said it's actually 17mm, ok, close enough) venturi to 18.5mm.

I've been tweaking some of the MOTA parameters so the absolute power numbers cannot be compared to my previous charts, only to the other runs on this chart.

Blue (mak84s21) is the mild woods port which I discussed in a previous post. 161 intake, 160 exhaust, 26 blowdown, 0.025 squish.

Red (mak84s23) is the bored out carb added to the mild woods port. The bigger carb did make more power on the top end, and it revved a little higher, too.

Violet (mak84s22) is the stock BB jug with no base gasket.

TW suggested that short stroke engines are very sensitive to squish, so I'm thinking about dropping it to 0.020". Doubt if squish velocity will be a problem, but the bore quality is already a problem. :msp_confused: Questionable bore extends down 0.200", so the ring seal may not be good. However, It may gain power nonetheless because more of the charge is burnt. Decisions, decisions.

I'm going to try a MOTA run with the lower transfers hogged out more. It may not be physically possible to actually remove more metal, but I just want to see if the lower transfers are a bottleneck. With the high base compression, they may not be.

 

[mtngun]

Here's MOTA's take on hogging out the lower transfers even more.

Blue (mak84s21) is the previously discussed mild woods port, which includes increasing the diameter of the lower transfers 1mm and the width of the upper transfers 1mm.

Red (mak84s24) starts with that mild woods port and hogs out the lower transfer diameters an ADDITIONAL 1mm (which may not even be physically possible :msp_tongue.

Hogging out the lower transfers didn't help one iota. They just doen't seem to be a bottleneck on this particular jug, if my MOTA-zation can be trusted. :msp_unsure:

One qualification for this test is that when I hogged out the lower transfers, I did not tell MOTA that the case compression had changed. In reality, hogging out the lower transfers does reduce the case compression slighty, which may be a bad thing.

 

[tdi-rick]

This is where programmes like this are at their absolute best, you can reconfigure whatever and run the results.

Absolute numbers aren't that important, it's the comparative results you are looking for.

The first graph shows what i would expect, the bigger venturi increases peak power and extends the rev range to the slight detriment of the torque peak.
In a situation like this the programme can validate what your gut/past experience suggests will happen, but where it's even better is when it throws up something you didn't expect, so then you really have to start thinking

Damn, I walked away from racing eleven or so years ago.
Too much money, too much time and wanted to be a more rounded human being, but this stuff just keeps nagging at me to play with.
I wonder what the Buddha would say

 

[mtngun]

This is where programmes like this are at their absolute best, you can reconfigure whatever and run the results.

I've already learned a thing or two from MOTA, like how one jug may be sensitive to the size of the transfer ports and another jug could care less.

I'm still puzzled about MOTA's odd way of measuring base compression, so I'll redo the base measurement for the 84cc jug this weekend and see if I can reconcile MOTA's method with Bell's method.