1970 1600 Roadster Top RPM's

[SPECIAL-K]

I have a 1970 1600 Roadster. A fresh engine was put in in 2021, carbs were just rebuilt and the distributor was replaced with a rebuilt, one with electronic points modified for the removal of the emission control gear. The engine runs great and cruises right along, the weird thing is, that in neutral if you rev the engine in zips right up to 6000 RPM's without hesitation, however, when in gear driving along in fourth gear the car will easily go up to 65 to 70 MPH, a little over 3000 RPM's and that's it. it runs fine and does not loose power even going up hills. It just doesn't want to go any more than that and it doesn't sound like it's straining. It's timed at 16 degrees BTDC per instruction and the vacuum advance is hooked up. I'm not sure if I've done something wrong, since everything seems to be working and sounding right. Since I didn't buy the car to race it and 65 to 70 is the speed limit maybe I should leave well enough alone. Any Thoughts


thanks

Keith

[fj20spl311]

There are so many possibilities
The first thing I would check is that the throttle is opening all the way.
then the polarity of the coil
is the coil "hot" after driving for a short time?

[Gregs672000]

How much total timing (do you have a dial back timing light)? Timing set with 0 being the left-most mark on the crank pully as reference, not the other way around? What do the spark plugs look like (post pics if you can). Ever done a compression or leak down test?

It absolutely should go much higher rpms in 4th! An engine can easily free rev but make no power.

[redroadster]

The centrifugal advance weights working on it ? Ign timing setting , never hurts to do a compression test, note 1st pump needs to be 65% of total 4 pumps possible carbs malfunction

[SPECIAL-K]

Thanks for the responses. I'll try to respond to all your responses right here. A compassion test showed 150 per cylinder straight across, the carbs were just rebuilt, balanced and synced, the plugs are fairly new show light gray. The engine stars right up, idles just fine and sounds great, has plenty of power right up till I get to about 70 or slightly above MPH in 4th gear. It doesn't cut out, sputter or try to slow down, it just doesn't want to go any farther. ??? I am not a real techy person so you will have to talk to me in plain English for me to understand complicated automotive stuff. I'm just a small sports car enthusiast that owned one of these cars back in the 70's and am reliving the fun.

thanks

Keith

[keith0alan]

Check the cam timing. With the crank at TDC either cylinder 1 or 4 will be in the center of the overlap where the exhaust is just closing and the intake just opening. if it isn't right in the center of the overlap then you are a tooth or two off on the timing chain. On the R16 it is a bit of a snot to fix. Second thought is to insure your exhaust isn't restricted. Does pulling the choke out make it better? this is a long shot but worth checking.

[redroadster]

Yeah and it runs out of speed in 1st at about 18 mph too ? It's diff ring gears ,are very low is why . My 70 1600 at 70 would be @6500 it's 52 00 @ 55- 58mph
I've got 14 75 185s on it when the car was built there wasn't highways in Asia or very few
More boulevards that were parking lots @rush hours , the car could stand a 3.0 ring gear

[Gregs672000]

Yes... does it rev to 6000rpm under load in 1st, 2nd, 3rd etc? As stated, it could be many things. Did it change when you switched to the EI dizzy, or was it possibly like this before? That will help you narrow it down if you're SURE.
150lbs is a little low but probably ok. Plugs sound good so less likely a fuel related issue, and you have confirmed carb flow and balance. At this point I'm leaning toward a spark timing issue, especially if it was good before. If you don't have one, go to Harbor Freight or similar and buy a dial back timing light. What this does is allow you to check the timing at idle (16 initial as you said, should flash between the two far right marks on the crank pully), and then as you increase the rpms the mark should move right, past the final right mark (20 degrees) and then hold steady at that point. With the dial on the timing light and with the rpms staying above 3000-3500, you should be able to "dial back" or adjust the flashing point to show 0 on the crank, and the dial should say it took 36 degrees of dial movement to accomplish this (kinda hard to describe). It may be that it dials back about 20 degrees (it's been a while since I've used mine) to add up to a total of 36 degrees at 3000+rpm (16 initial plus 20 added by the distributor)... I honestly can't recall, but bottom line is if it's not advancing enough it won't be able to make power. The fuel needs to be ignited progressively earlier in order to give it time to burn the fuel... too late and it just gets pushed out the exhaust, too early and it will cause the engine to fight itself (bad!). Check/set the timing with the vac advance line disconnected. Under higher load, the vac "advance" reduces timing a few degrees to avoid pre-ignition (distributor runs on mechanical advance only) then advances the timing off load to improve fuel economy... at least that's what I recall (I run a distributor-less system now and before did not run vac advance previously with a "solex" set up).

If it appears to be advancing normally (well past the final timing mark) then we may need to look further into cam timing issues as mentioned (seen it before). What do you know about the build on this Engine? Unfortunately it has come to light that there can be problems with cam timing when mixing up the metric vs SAE timing gears and cams (I believe that Curtis ran into this problem and there was much discussion). However, let's focus on simpler solutions first. Don't be afraid to ask ANY question or for clarification... we're here to help!