1975 GT185 versus 1974 GT185 Timing BTDC

[Vintageman]

There is a large difference in timing BTDC specification. Pre 1975 is 1.83mm and 1975 and on is 2.62mm.
Why?
Looking at Parts I see only one difference in engine. The Pre 1975 used a N5 Needle Jet and 1975 and on uses a N4. So slightly leaner.
Is this why the change or is there some other goal.
I have put some peaky expansion chamber on mine and wanted to retard timing a bit. Isn’t this what you should do if you plan to run higher RPM? I am thinking the 1.83mm

I may als0 raise exhaust port a tiny bit 0.75 mm and shave head about .5 mm, and take 0.75mm off piston skirt I hope to raise peak HP another 1000 to 1500 RPM only.

Thanks

[tz375]

I don't follow the logic. Why are the chambers peaky? Are they designed for an extremely high peak revs or do they just not match the ports well or are they lacking volume? Or is it just an assumption?

The only timing number that matters is the one that you find on a dyno that works best for your particular combination. Fully developed motors sometimes have more advance initially and modern ignitions retard that as revs rise because the effective compression ratio changes with engine load/RPM. A trick used for a few years now is to retard the very top end (after peak) to raise exhaust pipe temps to change the tuned length. Speed of sound rises with gas temperature. Hotter gas =higher sonic speed=higher speed at which teh pipe resonates=more power after the peak.

Timing on most of our old street 2 strokes was not very critical, but that's a huge difference. It could be that they were just too conservative initially or maybe they assumed that the bikes would run on low octane fuel and were conservative and subsequently changed their minds.

1.83 looks like it's around 19 degrees which you might expect to see on a motor turning 10,000 plus. 2.62 is roughly 23 degrees which is slightly more than the bigger bikes. (GT750 22+/-2). I'd say that the original number was conservative and the later one is a little much if your motor is ported and piped.

How did you work out those porting adjustments? Did you work through time area targets and run the answers through a simulator or at least did you compare the results to say A.G.Bell's target numbers? You might find that making ports wider gives you more time area without necessarily losing the bottom end as badly. I haven't worked the numbers on a 185 so that was speculation, but worth looking into.

Good luck it should be a fun bike. I didn't know that Jemco offered a pipe for that bike.

[Vintageman]

Tz375.

Per John Easton (JEMCO) these pipe were “all out” pipes specification defined by Suzuki back in the day. He lost his jig, but he knows how to cut up the sections. You get it as a kit and have to fit and weld a couple sections to your fitment style. much to do to make it fit, the foot pegs, break pedal etc. needs re thinking. Funny, I saved the center stand. These are not streat pipes like D&G for and RD for example

Here’s a YouTube of my bikehttp://www.youtube.com/watch?v=NJSIxujKfzk" onclick="window.open(this.href);return false;
Here’s a YouTube of someone else’s with Jemco’s http://www.youtube.com/watch?v=Evx1_iy9yVo" onclick="window.open(this.href);return false;
These pipes feel very Peaky in my opinion versus how the bike was before. My son says they don't kick in until 7500, but I see it hit at about 6500 up until 9000 where engines runs out of breadth not pipes I believe (why some minor porting maybe). Right now everything thing is stock
I am hoping to get another 1000 - 1500 RPM top. I was just assuming the bottom was gone forever and the top has more potential with these pipes (OK maybe I can widen exhaust port too?).

Don’t get me wrong it still runs slow, smoothly, but just won’t hold a hill for example like it did before. You rev it up above 7000 and it holds.
I think these pipes were meant to be road racer design. Under 6500 there is little torque. I mean the GT185 never really had a hit or peak before. With the pipes it sure does.

Porting … You said
“How did you work out those porting adjustments? Did you work through time area targets and run the answers through a simulator or at least did you compare the results to say A.G.Bell's target numbers? You might find that making ports wider gives you more time area without necessarily losing the bottom end as badly. I haven't worked the numbers on a 185 so that was speculation, but worth looking into.”
No clue what these means. I was thinking just raising the exhaust port up 1 mm. I just want a little more top end and I think the pipes are perfect for this and beyond. I wish I had some proven porting specs. I don’t want to road race. I just want a little more top end and if I could save the bottom great
But my original question. Why would Suzuki change the BTDC spec so much? Did they just change their mind and want different characteristics from the bike versus 1974 and earlier. I don't have a dyno so I am not sure which is better. I also agree early spec makes more sense. I am just confused as to why such a difference. Why would they change? I am an electrical engineer so why I question such things we do stuff for reason regardless of Marketing motivation

[Vintageman]

Ops wrong link to my bike. This should one work

http://www.youtube.com/watch?v=NJSIxujKfzk" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;

Please see my previous reply.

[jabcb]

Nice upgrade to your GT250: T350 top end & RD400 expansion chambers.
http://www.youtube.com/watch?v=n9XVfI2y8Zk

What modifications did you do to the chambers? How did it work out?

[tz375]

That's a nice little 185 and it sounds great too. I cannot believe how cleanly it runs down to almost no speed at all. That's good.

Of course the 185 never had much bottom end to begin with and from the little I can see of the pipes, they appear to be relatively large volume with steep angles and that makes for a good top end but not so good lower down. They look almost modern by comparison to most pipes back in that era. The large volume might help spread power compared to old skinny pipes but it's still going to be higher up than stock.

Ports
That's a little clearer now. What I was asking earlier was how you had calculated the proposed changes to ports. I wondered if you had used software or were following ideas from back in the day, but I understand that you are proposing to make a minor change and hope it will work. That's how we used to do it back in the day. I ran some numbers last night and that motor is theoretically capable of pushing out around 30HP but it would be a lot less rideable than it is now.

Raising the exhaust port will probably make it a better high rpm match for the pipe but that will lower the effective compression ratio when it's not on the pipe and that hurts bottom end too.

You could try a couple of things to try to add back a little at the lower end of the powerband. Making the pipes longer would drop the peak power and might help lower down. Slightly more advance would help the bottom end but could be detrimental further up the rev range.

The best thing to do is to raise the compression and that is what I'd do. That bike has a one piece head IIRC and it also has a squish band design. I'd measure squish clearance and angle and work out how much to take off the head or barrels to get the squish band down and to get compression up. I'd look at all the ports and work out what else could be modified without losing even more low down. Dropping the intake floor or shortening the piston all hurt the bottom end and typically more than they help at the top. Check the intake at TDC and see if the piston skirt s higher than the roof of the intake. If it is, raise the roof slightly. and widen it a little.

You could also widen the exhaust but be careful when widening ports to make sure that piston ring end can't get trapped in there.

[Vintageman]

tz375

I'll try to get a pic of just the chambers. They are short, the angles are steep, and pretty large volume too IMO.
In the video, near the end I shift from 2nd to 3rd. I did not over rev in 2nd and probably 7500 or maybe even 8000 when I shifted. At first if you notice 3rd almost didn't pull away for it dropped too much rpms when shifted from second, but since it was a straight it did make it and started to get on the pipe nicely. The gear spread is wide here.
If I had to pick only one, I would just like some bottom end back. But to me another 1000 rpm more is perfect for those pipes too.

Modifying those pipes for me is a @#$%. It was hard to fit them in the first place for it was a kit.
I think I'll raise the compression as you say. Last night I looked at some RD200 head gaskets. They are thin copper versus the thick paper with metal lip of the GT185. Maybe would be like shaving .012"/0.3mm. Maybe not enough.
How much would be a good start. (yes I'll check squish clearance, but just want to know what is a good start value)
I also thought about advancing timing for bottom. That's why I made this post. It just seemed so crazy different between the 74 to 75. The 75 as I have is very advanced now... I started to question going other way as maybe to why I cant get about 9000... if I can’t get bottom more rev is better so I can over rev if need be? I have RDs as well and those guys at usa2strokers swear you need to go more advance else holed pistons. Also the reason you said earlier about hotter exhaust at retard.
Oh well I diverge again and need to get back on track

So what do you think of this: A little more top and little more bottom, Can’t have both probably but…
1) I'll add use 2 or 3 base gaskets. This will raise the ports (0.030-0.040”… I’ll check once the come in). If no better I'll remove them. I'll check the intake as you said, but won't modify piston skirt. When I was young I had placed three base gaskets into a 1973 Yamaha DT250 (per a magazine article or something I read back when), that's all I did and it helped noticeably (sleazy, but easy to back out if no improvement)

2) I'll shave the head. Maybe .020" I'll account for extra base gaskets plus a bit more too start. Many heads on eBay versus jugs and much cheaper I mess up.
Now if I raise the port, shaving the head is not 1 to 1 since the head is Squish band. I mean if I raise the jugs 0.030" maybe shaving the head .015” returns to what it was?
Sorry about imperial units