NEED HELP 1978 Yamaha DT175

[Austin Endsley]

Having some issues with a bike I rebuilt looking for some thoughts.

1978 DT175

New - 

piston, rings, bore

Mikuni TM28 carburetor, VForce reed valves

crank seals, bottom end bearings

 

Tests-

crankcase holds 6 in/hg for over 10 minutes

115 lbs on compression check

exhaust is free of obstructions

Timing is set to stock marks

blue spark

 

Issue I'm having is bike starts and idles ( 2100rpm) however will not do much more than that. Also is running very rich, pull the plug after it runs and the plug is soaked. And getting some fuel sprayed back into the airbox. All carburetor adjustments make zero difference in how the bike runs. This is my first time posting, I have a video but unable to figure out if I can post it or not.

 

Thanks in Advance!

[cfaber]

Sounds like the float level is off

[Austin Endsley]

1 hour ago, cfaber said:

Sounds like the float level is off

I'm gonna mess with that tonight however I've had this issue for a while and have tried alot of different things including a second carb which I had the same issue with.

[cfaber]

If the float height is off, the static pressure on the jets will be off and metering won't be correct. Both carbs could share the same defect

[Austin Endsley]

Update- verified proper float height this evening. Also went ahead and tried the timing completely advanced then completely retarded. Both adjustments made zero difference in how the bike ran.

[CR-Dude4]

10 hours ago, cfaber said:

Sounds like the float level is off

There is a Canadian dude that restores some awesome Classics, he did a 82 yz 490 resto and he completly rebuilt the whole bike. He fired it up and he said it ran backwards. Check him out, hes amazing, He did fix the 490 and sold it. TNT Moto on youtube.

[Austin Endsley]

Update- verified proper float height this evening. Also went ahead and tried the timing completely advanced then completely retarded. Both adjustments made zero difference in how the bike ran.

[CR-Dude4]

1 minute ago, Austin Endsley said:

Update- verified proper float height this evening. Also went ahead and tried the timing completely advanced then completely retarded. Both adjustments made zero difference in how the bike ran.

Austin my man, time for the nitty gritty. Sounds like a cranky seal leak, possably worn out crank bearings and or rotating mass. Split da cases and go freshen up that bottom end.

[jkcanuck]

Austin;  You mentioned that the plug is wet; is it with fuel or oil?  If fuel, perhaps the starter-choke- plunger thingy isn't seating, allowing over-richness.  If Oil, when you replaced the primary-side crank seal, did you replace the o-ring behind the "distance collar" upon which the seal rides.? These are possibilities, however unlikely.  JK

[randysoo]

check the reeds. you mention fuel getting blown back into the airbox. that's not normal

[CR-Dude4]

2 minutes ago, randysoo said:

check the reeds. you mention fuel getting blown back into the airbox. that's not normal

Tired engine, its wise to freshen up the whole shebang before taking it out. 

[Pop's]

Carb slide in backwards? Cutout faces backward.

[Austin Endsley]

Update- I should have put this in the first post. Has new crank seals and new crank bearings. I did a top and bottom end. Tonight after several more hours reviewing my manual and the bike I found that the secondary ignition could was installed backwards basically. I flipped it around and bike fired right up now however it's idling around 5000 rpm and runs like it has an air leak. However the bike did respond to throttle and revved higher when I gave it some throttle twist. So I once again busted out my vacuum tester. The bike held 6in/hg again for over 6 minutes. I'm at all loss for that explanation but atleast I got the dirty non responsive idle issue figured out.

[Vettepilot]

"5000 rpm idle"  Had you previously turned idle up to keep it running?

Guys, he stated that it was all rebuilt and held a pressure test.  Actually reading a post before answering would be more helpful...

Vettepilot 

Edited June 28, 2024 by Vettepilot

[S.O.A.N.Z]

23 hours ago, Wally_Tunison_in_N.J. said:

Carb slide in backwards? Cutout faces backward.

 

[Jon houle]

5 hours ago, Vettepilot said:

"5000 rpm idle"  Had you previously turned idle up to keep it running?

Guys, he stated that it was all rebuilt and held a pressure test.  Actually reading a post before answering would be more helpful...

Vettepilot 

Check the air screw mixture it could be that check your timing see how close you are to top dead center maybe check your compression see if you have a head gasket leak it could maybe also be the floats perhaps the floats are not adjusted correctly 

 

[DOSHOW]

My first thought is what is your ring end gap?  I would think 115 compression is in the low side for a total rebuild?

[Austin Endsley]

Final Findings-

The majority if my problem was the incorrectly installed pickup coil. I'm assuming this retarded my timing by approximately 70-80 degrees. As for the high idleafter fixing the pickup coil, it was as simple as incorrect float level and turns out my new throttle cable is a little too short. Bike is back in a status that makes sense. I've got some final jetting to do but I'm well on my way to racing it in the hare scrambles at VMD this year. Thanks for the help

Edited July 3, 2024 by Austin Endsley
add info

[Vettepilot]

Very good!  Glad you got it sorted, and it wasn't anything bad.  Sometimes the simple stuff trips you up.  I bet it starts a bunch better/easier with proper timing and low throttle as well.

And THANKS for the follow up report!

Vettepilot 

Edited July 3, 2024 by Vettepilot