IT400 blues

[Colorado]

Hi all! New here, but have done a little pre-register perusing.

Maybe someone can help me out a little. I just got back into riding last year after a twenty-something year sabatical. I bought a '77 IT400. I grew up on 125's, and the 400 was a bit much for me at first. It was powerful and fast, and didn't stop that well besides. After a bit of riding, I started learning to handle it and was having a lot of fun. Then the rod threw through the piston skirt, cylinder skirt and both case halves.

I bought another IT400 a couple of weeks ago. It's a '78, and I thought --- hey! I've already got a parts bike! Well, the second one is NO where near as quick! It also has a loud piston rattle type sound at high RPM. I bought a Clymers and turned some wrenches and since figured out that the '77 actually had a 400 enduro bottom end with an IT400C top end (compression release). The pipe is also almost certainly not stock IT, as it has a separate head pipe. The '78 is a 400E.

I'm thinking that the IT400E should not be any tamer than the 400C, but thought I'd ask here. The power difference is so marked that I don't think cleaning and dialing in the fuel system will make up the difference. I also hoped someone might have a hint for me about the high RPM rattle. The intake manifold of the '78 is cracked looking, and I'm wondering about an air leak. The other one ran so strong that I thought I'd swap over the intake and reeds when I go through the carb because it couldn't hurt. I thought I'd like to check the compression before pulling the top end, but the Clymers doesn't give me any compression numbers. I also wondered what a primary compression leak would act like.

Do you guys think I'm on the right track?
Thanks in advance.

 

[Ol'89r]

I also hoped someone might have a hint for me about the high RPM rattle. The intake manifold of the '78 is cracked looking, and I'm wondering about an air leak.

Thanks in advance.

Hey Colorado, welcome to DRN.

If it were mine, I would pull the top end and mic everything. Sounds like the top end may be worn out. If you continue to run it with excessive clearance it could break a piston and cause a lot of damage.

An air leak in the intake manifold will cause the bike to rev without giving it throttle.

Not knowing anything about the pipe that is on the bike, you may want to go back to the stock pipe. I would put the bike back to as stock as possible and then start from there.

Just my $ .02

 

[Colorado]

Just my $ .02

Thanks! I posted the same message in General Repair and got pretty much the same payment. My own loose change involved pulling the jug before any real riding too, so six cents amounts to sound sense.

 

[placelast]

What the 'Ol guy said.

One of my first riding buds had one; it rattled at all RPMs, like my 175, only louder due to its size. Word had it all were like that but if you can hear it at a particular engine RPM and not others then there is a problem worth looking into.

At times my bud would be in front of me on our minute, and I could see it chuck small shovels of dirt at each power stroke, not unlike 4 strokers do; must have the sheer shovel-factor of that big mill plus the lower RPM at which the shoveling began relative to my wringer. On one of our B2V runs he ran across a dry lake bed in excess of 90mph; I peaked at a squeaky 75 as he left me in a cloud of dust.

Another fellow I knew had a '79, and his had a strange exhaust odor, said those who followed him. While apart, he mic'd it only to discover the barrel was misaligned relative to the rod, so he claimed. He said Yamaha wanted nothing of it.

The blue bikes were the bike to have at enduros in the late '70s. In the desert it was not uncommon to see them and Huskies be the two dominant choices (but a Maico 490 Spyder took the looks-cake! KTM was coming around, although rare.) Elsewhere I believe they could have benefitted with a steeper fork rake like the PEs to aid turning. Rumor had it there was a fellow in the SF Bay Area who had a jig to do a frame-head-tube mod, particularly useful for singletrack.

Other than the common rattle and this one barrel alignment they worked well.

 

[Colorado]

Good stories, Placelast. Before the first one blew up on me it was pretty cool for an old 125 man. When I did the 'throttle on and shift' thing I used to do on the small bores the front tire wear was minimal. It scared me a little at first!

Since it blew I've been through a TT500 and now also have a KDX175. I probably never would have thought of the KDX as pipey if I hadn't had that other IT400. I guess size really does matter.

 

[SFO]

Rumor had it there was a fellow in the SF Bay Area who had a jig to do a frame-head-tube mod, particularly useful for singletrack.

.

I know of two such shops that are able to still do this mod in SF.
Although Kosman a former employer has moved to Windsor Ca.