[md86]

Time to get a manual and start digging in .....
If coils are good , let's move on .
What did the other plugs look like ? Air filter ? All wires tight ? Try different plugs/check gap ? No hoses kinked ?

[80GS750]

Here's an update:

Last Sunday I found a 1-4 coil on a 1991 750 parts bike I have (I thought I had removed and tested it before, but had not), and it tested at 30.3k ohms on the secondary. I installed it, and the bike ran better, but still had an occasional afterfire. I took it for about a 10 mile ride. After it warmed up, the afterfire got worse, and it was pretty much flat above 5,000 rpm.

A couple of days later, I decided to drain the gas that I put in just before the problems started. I put more gas in from a different source, but still the afterfire remains.

I just finished checking the valves, and they were all within spec. The spark plugs looked like this:

#1 - black with white ring on end of porcelain (around the center electrode)

#2 - black

#3 - black

#4 - black with ends of side electrodes partially shiny

The air filter is clean.

Anybody have any ideas based on this new info? Thanks.

Wade

[Natoh786]

http://www.ngksparkplugs.com/techinfo/spark_plugs/faq/faqread2.asp?nav=31200&country=US

[80GS750]

I was told that my main jets may be too small. How much larger than stock should I go with just the Yoshimura header and stock airbox? Thanks.

Wade

[md86]

DJ116's or maybe 120's . But this is if you have DJ needles in there . You'll know this by looking at the needle . If it has notches , they're DJ .

[80GS750]

Not much progress here. I discovered tonight that my "mechanic" lied to me, and stole my stock main jets from a spare set of carbs. He was supposed to change the 144 mains that were in the bike when I bought it (and discovered tonight that they are still there) with the stock jets from the spare carbs. He gave me a sandwich bag of parts that were supposed to be the ones he removed (I didn't look at the contents when I picked up the bike, and only saw this a few days ago), but the mains weren't there. There was one tiny jet that looked like a pilot jet from something. He seemed "baffled" when I confronted him with this. I let him off the hook because I thought maybe he had lost them, but I'm going back tomorrow to insist he give me a set of stock main for this bike. I thought it was odd at the time that he didn't just exchange the jets in the carbs (put the stock ones in the good carbs, and put the 144s in the spare set). Also, when I opened up the spare set, I had to use an impact driver to remove two of the screws. Carbs that have recently been worked on don't require an impact driver to take apart.

Last night I checked the compression on all the cylinders, and they are all within specs (#1-150 lb., #2-157 lb., #3-144 lb., #4-158 lb.). When I started the bike the day before, #4 did not run, or was barely running (the exhaust pipe was barely lukewarm while the other 3 were hot). #4 plug was dry & fairly clean. It was still afterfiring.

Tonight when I took the carbs apart, I found that the rubber plugs over the pilot jets are loose, so I'm going to buy new ones. I'm also going to change the main jets back to stock before I put it back together.

I just bought a stock set of pipes on eBay for $1.29 plus $50 shipping. They are in really nice condition. Trouble is, they didn't come with any mounting hardware, so I may have to buy it new (ouch!). Anybody have any stock mounting hardware for sale?

Wade

[md86]

Mounting hardware for a stock pipe ? You SHOULD be able to use the header bolts you already got , and the bolts that go through the mufflers are easy enough to find at any hardware store . Just find a carriage bolt/nut about 1.5-2" long that fits through the hole back there . That's all I did .

[80GS750]

I have the header bolts, but not the clamps ($18.04 ea. x 4 at BikeBandit) and plates ($1.96 x 8) where the pipes attach to the engine. I don't have those. That's why I said "ouch" in my previous post. Do you have a low-cost substitute for those?

Wade

[ctandc]

Depends on how long you want to wait?

I just scored a post 98 Scorpion slip on for my 91, and will be swapping the post 98 oil pan / pick up and exhaust shortly. So maybe a week or so? I will send you all the stock mounting stuff for a little bit of nothing......heck I'd sell you my complete 91 exhaust in good shape /w header, and all bolts / mounting for $20 + actual shipping

HTH

Chris

[md86]

I have the header bolts, but not the clamps ($18.04 ea. x 4 at BikeBandit) and plates ($1.96 x 8) where the pipes attach to the engine. I don't have those. That's why I said "ouch" in my previous post. Do you have a low-cost substitute for those?

Wade

Oh , duh , yeah , I forgot about those parts . No , gotta get them, from somewhere . I can't see rigging something to work there . Maybe hit ebay or ask people parting stuff out for those parts .

[80GS750]

Depends on how long you want to wait?

I just scored a post 98 Scorpion slip on for my 91, and will be swapping the post 98 oil pan / pick up and exhaust shortly. So maybe a week or so? I will send you all the stock mounting stuff for a little bit of nothing......heck I'd sell you my complete 91 exhaust in good shape /w header, and all bolts / mounting for $20 + actual shipping

HTH

Chris

Chris:

I called the guy I bought the stock system from last night. He is looking for the hardware, and is supposed to let me know if he finds it today or Friday. If he doesn't have it, I'll get back with you. I'm not in a big hurry. I've got plenty of other stuff to work on! Thanks.

Wade

[80GS750]

I'm back working on the Kat again. I got frustrated working on it, and switched over to a 1980 Yamaha XS650 that I had sitting in the garage for over a year. I got it running after chasing down many problems, so I'm back to the Kat.

I discovered that the lower spring in the linkage between the #3 & #4 carbs was missing, causing #4 to not open until the other cylinders were about half open. I replaced it, then "bench synced" the carbs (all four were open a different amount at closed throttle) before putting them back on. I also have replaced the main jets with new stock jets, and replaced the pilot jet plugs with new ones.

I replaced the Yoshimura header this afternoon with the stock system, after I finally had to buy the clamps from the dealer at $18.50 each. I couldn't find them used anywhere, and got tired of looking. I now know why people throw away the stock hardware when installing a header. What a PITA it is to install with all the pieces!

It started up fairly easily, using the choke (enrichener). I noticed that #3 & #4 don't heat up nearly as quickly or as much as #1 & #2. It still has the nasty afterfire from one or both of those cylinders. I didn't get a chance to look at the plugs today, but will check them tomorrow. I have a new set to install. I'm afraid, though, that the carbs are going to have to come off again. After removing/reinstalling the carbs on the Yamaha at least 6 times, I'm really tired of working on carbs.

80GS750