irocbsa

My Feedback: (12)

Hi gents, I'm going round and round trying to make a decision for a new carb on an MT-35 I have sitting around. The carb has a lot of the typical problems that the Chinese "who-flung-poo" brand carbs do. I'm just going to replace it and keep the carb body for parts. I have a ton of genuine Walbros sitting around, but I'm having trouble deciding on the right part to use. In your opinion(s), would an 11.11mm or a 13.49mm venturi work better with this engine? I briefly fired it up with a WT-347 that has the 11.11mm venturi, but I had to set the needles close to 4.5 turns out to get it tuned. I realize this isn't the end of the world if it runs correctly at these settings, but I'm concerned that I'm giving up some power.

A lot of the 30cc engines use a 12.7mm venturi carb, but I have found a few people that say the 12.7 is a bit too much and they picked up power and a had a more reliable idle when they went to an 11.11mm carb. Of course, this engine has 5 more cc's, so 12.7 or 13.49 might work well. The 35cc displacement has been a bit of an odd ball until recently, and there isn't a consensus as to what works best on them. Now you see my dilemma. Thoughts?

ahicks

My Feedback: (2)

Nothing specific, just 2 thoughts. First, throttle response will be better with a smaller carb throat. The airflow through it will be faster. Second, remember how far that throttle arm moves towards the end of it's travel - with NO appreciable difference in rpm? That might give you something to consider when trying to figure out if you're going to be leaving any available power on the table.... My take, FWIW, -Al

flyinwalenda

My Feedback: (5)

I'm no expert but when I was hopping up some 31cc engines I tried both 11 and 12 mm venturi carbs. Without doing anything else to the engine the 11mm helped it out but the 12mm was too much. After doing work to the reed valve, polishing the ports and enlarging/dropping the intake,lowering the head, then the 12mm  worked great.
I would say if you want to run a 12-13mm carb you may have to make some of those modifications to the engine as well . Also try and find a Walbro model with an internal accelerator pump.

irocbsa

My Feedback: (12)

Thanks gents, and good point on the smaller throat Al. From my hot rodding days, the biggest mistake I remember guys making was putting on a carb that was too large for the engine. I recall running into guys with nearly stock 350 Chevys that had a big honkin' Holley 850 stuck on top. I'm going to try the 11mm WT-347 and see if I can make it work. I'll try to get some tach readings this week and post them.

ahicks

My Feedback: (2)

I'm going back into early days of sledding, when we would take one carb and it's manifold off a twin, and install 2 carbs. Hard to get dialed in, and no/very little gain at WOT. Bad plan....

irocbsa

My Feedback: (12)

Ha, I reckon it wouldn't.

Regarding the accelerator pump, I have read conflicting reports on using them in RC applications. None of the well running gassers that I have use them (EME55, DLE30, XYZ20), so if the manufacturer doesn't think the engine needs the accelerator pump, what's the case for using one?

flyinwalenda

My Feedback: (5)

I was told that if you have an engine that is difficult to tune on the low side changing out the carb with one that has a pump solves the problem. Also if the engine has had any internal modifications done to it the pump will help with tuning.
Seems to work for me.

irocbsa

My Feedback: (12)

Interesting. I'll try to tune it with a standard carb first since that's what I have. Thanks for the info!

irocbsa

My Feedback: (12)

I was able to get the engine running reasonably well on the wt-347 carb with the 11.11mm venturi. I'll post some tach numbers up when I run it at the field on Saturday. The high speed needle is at a typical setting of 2.25 turns, but the low speed is all the way out to 4.5 turns. I'm a little baffled, but that's what the engine wants.