Suzuki DT75 runs too lean

[scatter]

I'm trying to get a Suzuki DT75 (1989 I think) running for a friend. I "boiled" out the carbs and installed new float needle and seats. Got it running OK, and it idles fine. Problem is that when you raise the smaller throttle lever and bring it up to around 2000 RPMs it starts running lean. Coughs, almost dies and then back up to speed. Does this every 10 seconds or so. Runs OK with about 1/3 choke. When I ground out the top plug it basicly won't run. When I ground out either the middle or lower plugs it really doesn't make much difference in the way it runs. I'm going to check the compression, and fuel pump diaphram, but was wondering if anyone has any ideas. I'm stuck (and loving it) in Baja and can't get a manual till next trip out. Thanx for any help

 

[jim dozier]

Re: Suzuki DT75 runs too lean

What did you "boil" the carbs with. If you boiled it in water its probably still dirty. If you boiled it in solvent I'm calling the fire marshall! Seriously, it may be that you need to go back into the carbs. The Mikuni carbs on your Suzuki need to be kept very clean (not the outside, the orifices are what count). Disassemble them completely, clean with spray carb cleaner AND blow out with compressed air. Install a new carb kit and set the float height level with the bottom of the carb while holding the carb upside down. Set the idle air screws to about 1.25 turns out from lightly seated. Screw the idle air jet in to enrichen it. A manual will have the proper specs. You also need to synchronize the timing and carb linkages and you probably need a manual for that. I used to have a manual but I sold mine with the motor. Also check for air leaks in the fuel system.

 

[scatter]

Re: Suzuki DT75 runs too lean

Thanks for the reply Jimd! I "boiled" (a term we've used for over forty years that I know of) in a commerical carb dip, cleaner, no heat involved. I disassembled them, soaked 'em in the cleaner for a half hour, washed 'em off with water and blew all passages out with compressed air. Checked and made sure all passages in the jets were open and clean. I installed new gaskets and float needle and seat. I wasn't able to set the float height as had no manual so left them as they were. Because it is running lean, I checked for air leaks behind the carbs on the motor and in the fuel lines. I still haven't had a chance to open the fuel pump to check the diaphram, probably today. You say I screw the idle air screws in (clockwise) to richen the mixture? These are the ones right above the carb bore on the front of the carb? Anyway, it idles fine, it's the high speed where it seems to be running way too lean. I still don't understand the results I get when shorting out the different spark plugs, but I'll get into it again. Any ideas appreciated as we're a long way from anywhere and it seems as if I'm the resident expert. Yeah I know, we're in a world of hurt....

 

[jim dozier]

Re: Suzuki DT75 runs too lean

Well if one or more cylinders aren't carrying their weight it could be ignition or fuel. Cylinders won't fire if there is no spark and they won't fire if there is an incorrect mixture. The 1/3 choke would point to a lean condition as you have suggested. Have you tried pumping the fuel bulb? If that helps your fuel pump diaphragm needs replacement. If its not that or an air leak I would think its gotta be the carbs or your synchronization with the ignitin timing. Maybe the floats are way off. Maybe Rodbolt is watching and will step in here.